How do I calculate the heat loss of a room?

Add up the heat lost through each surface and through air leakage. For every wall, window, door, ceiling and floor, heat loss in BTU/h = U-factor × area in sq ft × the temperature difference in °F; for air leakage it is 0.018 × air changes per hour × room volume in cu ft × the temperature difference. For the 15 × 12 ft example (10°F outside, 70°F inside) that comes to 3,393 BTU/h: walls 943 BTU/h, windows 900 BTU/h, ceiling 272 BTU/h, floor 500 BTU/h, air leakage 778 BTU/h.

What is the 0.018 in the air leakage formula?

It is the heat needed to warm one cubic foot of air by one degree Fahrenheit, in BTU: roughly 0.075 lb of air per cubic foot times a specific heat of 0.24 BTU per lb·°F. Heating-load texts print the infiltration formula as Q = 0.018 × ACH × volume × ΔT, the ACH being the number of times an hour the room’s air is replaced. At high altitude, where the air is thinner, the figure is lower. It does not change for the room size or the climate.

What outdoor design temperature should I use?

Use the 99% heating design temperature for your location. It is the temperature that the outdoors stays above for 99% of the hours of a year, and it is published in ASHRAE climate data and in ACCA Manual J Table 1A (a contractor, a building department or an energy-code appendix will have it for your city). It is not the record low. This page does not include a city table, so you enter it. ACCA’s guidance is to use the published value as it is and not add a safety factor. Going from 10°F to -10°F on the example room raises the heat loss from 3,393 BTU/h to 4,524 BTU/h.

What indoor temperature should I use?

ACCA Manual J’s standard indoor design condition for heating is 70°F, so that is the starting value here. Use another figure only if the room really is kept at another setpoint. Each extra degree of indoor temperature adds the same amount as one degree colder outdoors.

What is a good U-factor for a window?

Lower is better. Typical whole-window values are about 1.1 for clear single pane, 0.5 for clear double pane and 0.3 for low-E double pane with argon. ENERGY STAR Version 7.0 (effective October 2023) allows a maximum of 0.22 for a window in its Northern zone, 0.25 in North-Central, 0.28 in South-Central and 0.32 in the Southern zone, with tighter limits than the earlier Version 6. In the example room, 30 sq ft of windows costs 1,980 BTU/h as single pane, 900 BTU/h as double pane and 396 BTU/h at the Version 7 Northern limit. Use the U-factor on your window’s NFRC label if you have it.

How does insulation in the walls, attic and floor change the answer?

A higher R-value lowers the U-factor and so the loss, but each step gives less: the loss through the ceiling is U × area × ΔT, and U = 1 ÷ (R + 1.67), so going from R-0 to R-19 saves much more than R-19 to R-38. In the example room the R-38 attic ceiling loses 272 BTU/h; with no attic insulation it would lose 6,467 BTU/h and the room would need 9,587 BTU/h. Walls are different again: the wall figure comes from the wall R-value calculator, which allows for the studs, so a 2×6 wall at 16 in on center with R-21 batts (U-0.0585, R-17.11) takes the example from 3,393 BTU/h to 3,102 BTU/h.

How much does air leakage matter?

A lot in a drafty house. At 0.5 air changes per hour the example room loses 778 BTU/h to leakage, 23% of its total; at 0.25 ACH the room needs 3,004 BTU/h, and at 1.5 ACH it needs 4,948 BTU/h. Sealing leaks around windows, doors, outlets and the attic floor is often the cheapest way to lower a heat loss. Use a natural ACH, not a blower-door ACH50 figure; ACH50 divided by about 17 is a common rough conversion in cold climates.

How do I convert BTU/h to watts, and what is W/m²?

One BTU/h is 0.29307 watts, so 3,393 BTU/h is 994 W (0.99 kW, since 1 kW is 3,412 BTU/h). Dividing the watts by the floor area in square metres gives the heat loss per m² of floor, which is how many European tools and radiator guides express it: 59 W/m² for the 15 × 12 ft example, which is 16.7 m².

Is this a Manual J calculation?

No. Manual J, published by ACCA, is the standard method for sizing heating and cooling equipment, and it is what a contractor or code official means by a load calculation. It treats each room and the whole house, includes duct losses, ventilation, internal and solar gains, and uses detailed construction data and local climate data. This page applies the same Q = U × A × ΔT idea to a single room with simple inputs, so it is good for finding which surfaces lose the most heat and for checking a quote, but it should not be the basis for choosing a furnace, boiler or heat pump. Ask for a Manual J.

How does this compare with the furnace size rule of thumb?

The furnace size calculator multiplies the floor area by a typical BTU/h per sq ft for a climate zone. For the 180 sq ft example in zone 5 that gives 9,000 BTU/h, against 3,393 BTU/h here from the walls, windows and air leakage. A single room with two outside walls loses less than the whole-house average per square foot, and the rule of thumb also allows for ducts, extra exposure and older construction. The two numbers answer different questions: the rule is a quick ballpark for a whole home (see the furnace size calculator); this page is a component-by-component estimate for one room. Neither replaces a Manual J.

Why use the full temperature difference for the floor?

To keep the estimate simple and on the safe side. The air under a floor over a crawl space or an unheated basement is usually warmer than the outdoor design temperature, so the real loss is lower. Manual J uses a ground or crawl-space temperature for this. Pick “Slab or heated space below” if the floor is on a slab or over a heated room, where this page counts nothing; a slab edge and a below-grade wall are not covered here.

What if the room has no outside walls or is an interior room?

Set the exterior wall length to 0 and the windows and doors to 0. A room between heated rooms loses almost nothing through its walls. If it is also between floors (a heated space above and below), the ceiling and floor choices “heated space above” and “slab or heated space below” count nothing; the example room with those two surfaces removed needs 2,621 BTU/h instead of 3,393 BTU/h.

Room Heat Loss Calculator: Design Heat Loss in BTU/h

A 15 × 12 ft room with 8 ft ceilings, 27 ft of exterior wall (2×4 studs, R-13 batts), 30 sq ft of double-pane windows, an R-38 attic above and an R-19 floor over a crawl space loses about 3,393 BTU/h on a 10°F design day with the house held at 70°F (a 60°F difference), which is 994 W or 59 W/m² of floor. The walls account for 943 BTU/h, the windows 900 BTU/h, the ceiling 272 BTU/h, the floor 500 BTU/h and air leakage 778 BTU/h. Enter your own outdoor design temperature and construction to see each part. This is a simplified estimate of one room, not a Manual J load calculation; the rule of thumb on the furnace size page would give 9,000 BTU/h for the same floor area, and Manual J is how heating equipment should really be sized.

Figures based on the sources listed belowUpdated 2026-10-18How we calculate

Room length

Room width

Outdoor design temperature (°F)

The 99% heating design dry-bulb temperature for your location: the temperature it stays above for 99% of the hours of a year. Look it up in ACCA Manual J Table 1A or ASHRAE climate data (your contractor or building department has it); do not use the all-time record low.

Indoor design temperature (°F)

ACCA Manual J’s standard indoor condition for heating is 70°F.

Ceiling height

Sets the wall height and the room volume for air leakage. The room length and width above give the floor and ceiling area.

Exterior wall length

The total length of the room’s walls that face outdoors (not walls shared with other heated rooms). A corner room with two outside walls is its length plus its width.

Wall framingThe wall’s framing, as in the wall R-value calculator: sets the cavity depth and the share of the wall that is wood (25% at 16 in on center, 22% at 24 in).

Cavity insulationBatts use their rated R-value; blown and sprayed fills use R per inch over the full cavity depth. A batt thicker than the cavity is scaled down. Typical figures: products vary.

Interior finishThe layer on the inside face of the studs.

SheathingStructural sheathing on the outside face of the studs.

Continuous exterior foamRigid foam board over the sheathing. It covers the studs too, so it adds its full R-value to both paths and is the usual way to beat the framing penalty.

CladdingThe outer skin. Siding adds little; brick is better but is usually spaced off the wall by an air gap that is not counted here.

Window area (sq ft)

The total area of every window in the exterior walls, frame included (a 3 × 4 ft window is 12 sq ft). It comes out of the wall area.

Window typeWhole-window U-factors as on an NFRC label. Typical figures: your window’s own label is better.

Exterior doors

The number of doors to outdoors, each counted as a 3 × 7 ft (21 sq ft) slab. A glass door can be entered as windows instead.

Door type

CeilingAttic insulation R-value. Ceiling U-factor = 1 ÷ (R + 1.67), the 1.67 being the air films and drywall; joist framing is ignored.

FloorInsulation between the joists over a crawl space, garage or open air. Floor U-factor = 1 ÷ (R + 2.61), the 2.61 being the air films and a plywood subfloor. A slab on grade or a floor over a heated room is not counted.

Air changes per hour (natural, ACH)

How many times an hour the room’s air is replaced by outdoor air at design conditions. About 0.15 to 0.5 for a tight, newer house, 0.8 to 1.5 or more for an older, leaky one. Do not enter a blower-door ACH50 figure here; divide it by about 17 first.

Compare with the furnace size rule of thumbThe furnace size calculator’s typical BTU/h per sq ft for an IECC / DOE climate zone, applied to this room’s floor area.

3,393 BTU/h

Exterior walls: 943 BTU/h (186 sq ft net of windows and doors × U-0.0845, R-11.83 × 60°F; 28% of the total)
Windows: 900 BTU/h (30 sq ft × U-0.5, double-pane window × 60°F; 27% of the total)
Exterior doors: 0 BTU/h (0 sq ft × U-0.2, insulated door × 60°F; 0% of the total)
Ceiling under attic: 272 BTU/h (180 sq ft × U-0.025 × 60°F; 8% of the total)
Floor over unheated space: 500 BTU/h (180 sq ft × U-0.046 × 60°F, taking the full outdoor temperature difference; 15% of the total)
Air leakage: 778 BTU/h (0.018 × 0.5 air changes per hour × 1,440 cu ft × 60°F; 23% of the total)
Total design heat loss: 3,393 BTU/h at 70°F inside and 10°F outside (a 60°F difference)
In watts: 994 W (0.99 kW), 59 W/m² of floor
Furnace size rule of thumb, same room: 9,000 BTU/h (zone 5: 180 sq ft × 50 BTU/h per sq ft). This room’s estimate is 62% lower. A square-foot rule sees none of the walls, windows or air leakage; Manual J is the real method

Shopping list

Weatherstripping
Window insulation film kit
Caulk for air sealing
Infrared thermometer
Tape measure (25 ft)

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Window and door U-factors used here, and the loss per 10 sq ft

Whole-product U-factors in BTU/h·ft²·°F. The last-but-one column is the heat loss through 10 sq ft on a design day with a 60°F difference (70°F inside, 10°F outside).
Kind	Type	U-factor	Loss per 10 sq ft at 60°F	Basis
Window	Single pane, clear	1.1	660 BTU/h	Typical; DOE puts clear single glass near 1.07
Window	Double pane, clear	0.5	300 BTU/h	Typical; clear double glazing runs about 0.45 to 0.55
Window	Double pane, low-E, argon	0.3	180 BTU/h	Typical; low-E argon units run about 0.25 to 0.35
Window	ENERGY STAR v7 Northern zone	0.22	132 BTU/h	ENERGY STAR Version 7.0 maximum for the Northern zone
Door	Wood	0.5	300 BTU/h	Code default U-factor for a wood door
Door	Insulated steel or fiberglass	0.2	120 BTU/h	Typical; ratings run about 0.15 to 0.35
Door	Uninsulated metal	1.2	720 BTU/h	Code default U-factor for an uninsulated metal door

Ceiling and floor U-factors by insulation R-value, and the loss per 100 sq ft

U = 1 ÷ (R + the surface adder): 1.67 for a ceiling under an attic and 2.61 for a floor over an unheated space. Loss per 100 sq ft at a 60°F difference. Framing is ignored.
Insulation	Ceiling U	Ceiling loss per 100 sq ft	Floor U	Floor loss per 100 sq ft
R-0	0.599	3,593 BTU/h	0.383	2,299 BTU/h
R-11	0.079	474 BTU/h	0.073	441 BTU/h
R-19	0.048	290 BTU/h	0.046	278 BTU/h
R-30	0.032	189 BTU/h	0.031	184 BTU/h
R-38	0.025	151 BTU/h	0.025	148 BTU/h
R-49	0.02	118 BTU/h	0.019	116 BTU/h

Air leakage loss for a 15 × 12 ft room with 8 ft ceilings, by air changes per hour

1,440 cu ft of air at a 60°F difference: 0.018 × ACH × volume × ΔT. Typical natural air-change rates; a blower-door ACH50 figure is not the same thing.
ACH	Typical of	Air leakage loss
0.15	very tight new house (low end of 0.15 to 0.5)	233 BTU/h
0.25	tight, newer construction	389 BTU/h
0.5	typical newer house, and this page’s starting value	778 BTU/h
1	average older house	1,555 BTU/h
1.5	leaky older house (high end of 0.8 to 1.5)	2,333 BTU/h

The example room’s heat loss by outdoor design temperature

The same 15 × 12 ft room as the page’s starting example with the house held at 70°F; only the outdoor design temperature changes. Use the 99% design temperature for your location, not an example from this table.
Outdoor design temperature	Temperature difference	Total heat loss	In watts
40°F	30°F	1,696 BTU/h	497 W
30°F	40°F	2,262 BTU/h	663 W
20°F	50°F	2,827 BTU/h	829 W
10°F	60°F	3,393 BTU/h	994 W
0°F	70°F	3,958 BTU/h	1,160 W
-10°F	80°F	4,524 BTU/h	1,326 W

Methodology & assumptions

This is a simplified, steady-state estimate of the heat a single room loses on a design day, in the spirit of ACCA Manual J but not Manual J. Manual J, published by ACCA (the Air Conditioning Contractors of America), is the method contractors use to size heating and cooling equipment: it also counts thermal bridging, duct losses, solar gain, internal gains, ventilation and the real construction of each surface. Heat flows through each surface in proportion to its area, its U-factor and the temperature difference ΔT, so Q = U × A × ΔT in BTU/h, with U in BTU/h·ft²·°F (the inverse of the R-value). The steps are as follows. 1) ΔT = indoor design temperature − outdoor design temperature. Manual J’s standard indoor condition for heating is 70°F. The outdoor value is the 99% heating design dry-bulb temperature for your location, which ASHRAE’s climate data and ACCA Manual J Table 1A publish: the temperature the outdoors stays above for 99% of the hours of a year. You enter it yourself, because this site does not include a city table; use the published value as it stands and add no safety factor of your own. 2) Exterior walls: net area = exterior wall length × ceiling height − window area − door area, so the openings are not counted twice. The wall U-factor comes straight from the wall R-value calculator on this site, a parallel-path calculation of the studs and the insulation between them with the same assembly choices (framing, cavity insulation, interior finish, sheathing, exterior foam and cladding, with the inside and outside air films). 3) Windows: area × the window’s U-factor. Window U-factors are whole-window figures, frame included, as on an NFRC label; the typical values are single-pane window 1.1, double-pane window 0.5, low-E double-pane window 0.3, ENERGY STAR v7 Northern window 0.22. The ENERGY STAR Version 7.0 limit for the Northern zone is U-0.22 for a window. 4) Exterior doors: each is counted as 21 sq ft (3 × 7 ft) with the U-factor of its type (wood door 0.5, insulated door 0.2, uninsulated metal door 1.2). 5) Ceiling under an attic: floor area × U, with U = 1 ÷ (insulation R + 1.67). The 1.67 adds the inside air film (0.61 for heat flowing up), 1/2 in drywall (0.45) and the film on the attic side (0.61); heat bridging through the joists is ignored, which understates the loss a little. If another heated floor is above, nothing is counted. 6) Floor over an unheated space: floor area × U, with U = 1 ÷ (insulation R + 2.61), the extra being the inside air film for heat flowing down (0.92), a 5/8 in plywood subfloor (0.77) and the film beneath (0.92). The full outdoor temperature difference is used, which overstates the loss over a crawl space or basement, whose air is warmer than outdoors; a slab on grade, or a floor over a heated room, is not counted. 7) Air leakage: Q = 0.018 × ACH × room volume × ΔT. The constant 0.018 BTU/ft³·°F is the heat capacity of air per cubic foot: about 0.075 lb of air per cu ft times 0.24 BTU/lb·°F, the figure used in heating-load work; it is lower at high altitude. ACH is the natural air changes per hour at design conditions, entered by you: about 0.15 to 0.5 for a tight, newer house and 0.8 to 1.5 or more for an older one. Do not use a blower-door ACH50 figure: it is measured at a 50-pascal pressure and overstates everyday leakage, by a factor of about 17 in a cold climate. A room with a door to other heated rooms leaks less than the whole house per cubic foot, so this is an estimate; enter your own best figure. 8) The total is the sum, rounded to the nearest BTU/h. Watts = BTU/h × 0.29307, and W/m² divides the watts by the floor area in square metres (1 sq ft = 0.0929 m²). To size a heater, a radiator or a mini-split for the room, the total is the design-day heat it has to replace; add nothing here for a safety margin. Limits: it does not count heat gained from the sun, people or appliances, duct losses, ventilation fans, a slab on grade’s edge loss, below-grade walls, skylights, vaulted ceilings, thermal bridges at windows and doors, or heat that moves between rooms. The comparison with the furnace size rule of thumb applies that page’s typical BTU/h per sq ft figure for the climate zone you choose to this room’s floor area; it is a whole-home figure and is usually not a fair match for a single room, which is why the two numbers can differ widely. Where a value is labeled typical it is a typical figure that varies by product; use the maker’s figure or the label on your window when you have it.

ACCA HVAC Blog, Are You Making These Mistakes With Manual J?: ACCA’s own statement of the standard indoor conditions for Manual J, 70°F for heating and 75°F for cooling at 50% relative humidity, and the advice to change them only if the system is really run at another setpoint
ACCA, Outdoor Design Conditions guide (Manual J 8th Edition): how to read the 99% heating design temperature in Table 1A, and the instruction to use the published values without adding a safety factor. The 99% value is the minimum dry-bulb temperature a location is expected to stay above for 99% of the hours of a year, taken from ASHRAE climate data
BCcampus Open Textbooks, Plumbing 3 (Learning Task 4, Calculating transmission loss): the transmission loss formula Q = U × A × ΔT and the infiltration loss formula ACH × 0.018 × temperature difference × room volume
CED Engineering, Heat Loss Calculations and Principles (course M05-003): heat loss through walls, windows and ceilings as U × A × ΔT, the heat capacity of air of 0.018 BTU/ft³·°F in the infiltration formula, and typical winter air-change rates of about 0.15 to 0.5 per hour. The ASHRAE Handbook of Fundamentals is the underlying reference; this site could not open the Handbook itself, so ASHRAE is cited through these secondary texts
GreenBuildingAdvisor, ACH50 infiltration heat loss (Q&A): why a blower-door ACH50 figure must be divided (by about 17 in a cold climate) before use in the infiltration formula, since it is measured at 50 pascals and not at everyday pressures
ENERGY STAR, Residential Windows, Doors, and Skylights Version 7.0 final specification (effective October 23, 2023): maximum U-factors for windows by climate zone, Northern 0.22, North-Central 0.25, South-Central 0.28 and Southern 0.32, and lower limits for swinging doors. Checked from search result summaries of the specification; the 0.22 Northern window figure is the one used here
City of Seattle, Seattle Energy Code Appendix A (2021): default heat loss coefficients taken from ASHRAE, with air films of 0.68 (vertical), 0.61 (horizontal, heat flowing up), 0.92 (heat flowing down) and 0.17 (outside), and default door U-factors of 0.50 for wood and 1.20 for uninsulated metal. A code reproduction of ASHRAE values, not the Handbook itself
US Department of Energy, Selecting Windows for Energy Efficiency (as reproduced by the National Association of State Community Service Programs): about U-1.07 for clear single glass and about 0.48 for double glass with a 1/2 in air space. The U-0.30 low-E and U-0.50 clear double-pane figures on this page are typical values within the 0.25 to 0.35 and 0.45 to 0.55 ranges that window guides print, not a single published table
ACCA, Manual J, Residential Load Calculation: the standard method for sizing home heating and cooling equipment. This page is an estimate and not a Manual J calculation
The wall R-value calculator on this site (parallel-path method, ASHRAE framing fractions of 25% and 22%): the source of the wall U-factor. Its sources are listed on that page
Typical figures, not published tables: the insulated door U-0.20 (ratings run about 0.15 to 0.35), the 3 × 7 ft door size, the 0.5 air changes per hour starting value, and the ceiling and floor adders of 1.67 and 2.61, which are sums of the air film, drywall and plywood values above

How to estimate the heat loss of a room

Enter the design temperatures. Type the 99% heating design temperature for your location (from ACCA Manual J Table 1A or ASHRAE climate data) and the indoor temperature, 70°F by default. Their difference is ΔT.
Measure the room. Enter the floor length and width, the ceiling height and the total length of the walls that face outdoors. Floor and ceiling area come from the length and width; the wall area is the wall length times the ceiling height.
Describe the wall, windows and doors. Choose the wall construction, add up the window area and pick the window type, and count the exterior doors. Window and door areas are taken out of the wall area.
Choose the ceiling and floor. Pick the attic insulation R-value if there is an attic above, and the floor insulation if the floor is over an unheated space. Pick the “not counted” choices for a heated space above or below.
Enter the air changes per hour. About 0.5 for a typical newer house, 0.25 for a tight one and 1 or more for an older, leaky one. Do not use a blower-door ACH50 figure.
Read the total and check it with a Manual J. The total is the room’s design heat loss in BTU/h, with watts and W/m². Use it to compare options and spot the weak surfaces, and ask a contractor for a Manual J before sizing any equipment.

Worked examples

Project	Result
15 × 12 ft room, 10°F outside, 70°F inside, R-13 walls, double-pane windows, R-38 attic, R-19 floor, 0.5 ACH	3,393 BTU/h: walls 943 BTU/h, windows 900 BTU/h, ceiling 272 BTU/h, floor 500 BTU/h, air 778 BTU/h
The same room with single-pane windows	4,473 BTU/h, of which the windows are 1,980 BTU/h
The same room with ENERGY STAR Version 7 Northern zone windows (U-0.22)	2,889 BTU/h, the windows 396 BTU/h
The same room with 2×6 walls at 16 in and R-21 batts	3,102 BTU/h, the walls 652 BTU/h (R-17.11)
The same room with an uninsulated attic (R-0)	9,587 BTU/h, the ceiling 6,467 BTU/h
The same room with a leaky 1.5 air changes per hour	4,948 BTU/h, air leakage 2,333 BTU/h
The same room with one insulated exterior door (21 sq ft)	3,538 BTU/h, the door 252 BTU/h, the wall 837 BTU/h
The same room on a −10°F design day	4,524 BTU/h, a temperature difference of 80°F
The same room with heated space above and below	2,621 BTU/h, with no ceiling or floor loss

Frequently asked questions

How do I calculate the heat loss of a room?: Add up the heat lost through each surface and through air leakage. For every wall, window, door, ceiling and floor, heat loss in BTU/h = U-factor × area in sq ft × the temperature difference in °F; for air leakage it is 0.018 × air changes per hour × room volume in cu ft × the temperature difference. For the 15 × 12 ft example (10°F outside, 70°F inside) that comes to 3,393 BTU/h: walls 943 BTU/h, windows 900 BTU/h, ceiling 272 BTU/h, floor 500 BTU/h, air leakage 778 BTU/h.
What is the 0.018 in the air leakage formula?: It is the heat needed to warm one cubic foot of air by one degree Fahrenheit, in BTU: roughly 0.075 lb of air per cubic foot times a specific heat of 0.24 BTU per lb·°F. Heating-load texts print the infiltration formula as Q = 0.018 × ACH × volume × ΔT, the ACH being the number of times an hour the room’s air is replaced. At high altitude, where the air is thinner, the figure is lower. It does not change for the room size or the climate.
What outdoor design temperature should I use?: Use the 99% heating design temperature for your location. It is the temperature that the outdoors stays above for 99% of the hours of a year, and it is published in ASHRAE climate data and in ACCA Manual J Table 1A (a contractor, a building department or an energy-code appendix will have it for your city). It is not the record low. This page does not include a city table, so you enter it. ACCA’s guidance is to use the published value as it is and not add a safety factor. Going from 10°F to -10°F on the example room raises the heat loss from 3,393 BTU/h to 4,524 BTU/h.
What indoor temperature should I use?: ACCA Manual J’s standard indoor design condition for heating is 70°F, so that is the starting value here. Use another figure only if the room really is kept at another setpoint. Each extra degree of indoor temperature adds the same amount as one degree colder outdoors.
What is a good U-factor for a window?: Lower is better. Typical whole-window values are about 1.1 for clear single pane, 0.5 for clear double pane and 0.3 for low-E double pane with argon. ENERGY STAR Version 7.0 (effective October 2023) allows a maximum of 0.22 for a window in its Northern zone, 0.25 in North-Central, 0.28 in South-Central and 0.32 in the Southern zone, with tighter limits than the earlier Version 6. In the example room, 30 sq ft of windows costs 1,980 BTU/h as single pane, 900 BTU/h as double pane and 396 BTU/h at the Version 7 Northern limit. Use the U-factor on your window’s NFRC label if you have it.
How does insulation in the walls, attic and floor change the answer?: A higher R-value lowers the U-factor and so the loss, but each step gives less: the loss through the ceiling is U × area × ΔT, and U = 1 ÷ (R + 1.67), so going from R-0 to R-19 saves much more than R-19 to R-38. In the example room the R-38 attic ceiling loses 272 BTU/h; with no attic insulation it would lose 6,467 BTU/h and the room would need 9,587 BTU/h. Walls are different again: the wall figure comes from the wall R-value calculator, which allows for the studs, so a 2×6 wall at 16 in on center with R-21 batts (U-0.0585, R-17.11) takes the example from 3,393 BTU/h to 3,102 BTU/h.
How much does air leakage matter?: A lot in a drafty house. At 0.5 air changes per hour the example room loses 778 BTU/h to leakage, 23% of its total; at 0.25 ACH the room needs 3,004 BTU/h, and at 1.5 ACH it needs 4,948 BTU/h. Sealing leaks around windows, doors, outlets and the attic floor is often the cheapest way to lower a heat loss. Use a natural ACH, not a blower-door ACH50 figure; ACH50 divided by about 17 is a common rough conversion in cold climates.
How do I convert BTU/h to watts, and what is W/m²?: One BTU/h is 0.29307 watts, so 3,393 BTU/h is 994 W (0.99 kW, since 1 kW is 3,412 BTU/h). Dividing the watts by the floor area in square metres gives the heat loss per m² of floor, which is how many European tools and radiator guides express it: 59 W/m² for the 15 × 12 ft example, which is 16.7 m².
Is this a Manual J calculation?: No. Manual J, published by ACCA, is the standard method for sizing heating and cooling equipment, and it is what a contractor or code official means by a load calculation. It treats each room and the whole house, includes duct losses, ventilation, internal and solar gains, and uses detailed construction data and local climate data. This page applies the same Q = U × A × ΔT idea to a single room with simple inputs, so it is good for finding which surfaces lose the most heat and for checking a quote, but it should not be the basis for choosing a furnace, boiler or heat pump. Ask for a Manual J.
How does this compare with the furnace size rule of thumb?: The furnace size calculator multiplies the floor area by a typical BTU/h per sq ft for a climate zone. For the 180 sq ft example in zone 5 that gives 9,000 BTU/h, against 3,393 BTU/h here from the walls, windows and air leakage. A single room with two outside walls loses less than the whole-house average per square foot, and the rule of thumb also allows for ducts, extra exposure and older construction. The two numbers answer different questions: the rule is a quick ballpark for a whole home (see the furnace size calculator); this page is a component-by-component estimate for one room. Neither replaces a Manual J.
Why use the full temperature difference for the floor?: To keep the estimate simple and on the safe side. The air under a floor over a crawl space or an unheated basement is usually warmer than the outdoor design temperature, so the real loss is lower. Manual J uses a ground or crawl-space temperature for this. Pick “Slab or heated space below” if the floor is on a slab or over a heated room, where this page counts nothing; a slab edge and a below-grade wall are not covered here.
What if the room has no outside walls or is an interior room?: Set the exterior wall length to 0 and the windows and doors to 0. A room between heated rooms loses almost nothing through its walls. If it is also between floors (a heated space above and below), the ceiling and floor choices “heated space above” and “slab or heated space below” count nothing; the example room with those two surfaces removed needs 2,621 BTU/h instead of 3,393 BTU/h.