What size furnace do I need for my house?

Multiply the heated floor area by the BTU/hr per sq ft for your climate zone, adjust for insulation and ceiling height, then divide by the furnace’s AFUE. For 1,600 sq ft in zone 5 with average insulation that is 80,000 BTU/hr of heat, or 100,000 BTU/hr input at 80% AFUE, so the smallest typical size that covers it is 100,000 BTU/hr. It is a rule of thumb: a Manual J load calculation is the proper way to choose.

What is the difference between furnace input and output?

Input is the fuel the burner uses, in BTU/hr; output is the heat that reaches the home. Output = input × AFUE. A furnace labelled 100,000 BTU/hr input at 80% AFUE delivers 80,000 BTU/hr, and at 95% AFUE it delivers 95,000. Furnaces are named and sold by input, so the size you shop for is the output you need divided by AFUE: 80,000 BTU/hr of output is 100,000 BTU/hr input at 80% and 84,211 BTU/hr input at 95%.

AFUE, annual fuel utilization efficiency, is the US Department of Energy’s measure of the share of a furnace’s fuel energy that becomes heat for the home over a typical year. An 80% AFUE furnace turns 80% of the fuel into house heat and loses the rest up the flue; a 95% condensing furnace loses only 5%. DOE’s AFUE leaves out heat lost from ducts, which can be large in an attic or crawl space. The federal minimum for new gas furnaces is currently 80%, and DOE has set 95% for new non-weatherized gas furnaces from late 2028.

Is this a Manual J calculation?

No. Manual J, published by ACCA, is the method contractors use to size heating and cooling equipment. It counts insulation levels, window area and type, air leakage, ducts and the local design temperature for each room. This page uses one BTU per sq ft figure for a climate zone, so it can be off by 25% or more for an unusual house. Use it for a ballpark when budgeting or checking a quote, and ask your contractor for a Manual J.

How does insulation change the size?

This tool adds 10% for poor insulation and subtracts 10% for good insulation, the adjustment several HVAC guides use. For the 1,600 sq ft zone 5 home at 80% AFUE that is 110,000 BTU/hr input with poor insulation, 100,000 BTU/hr input with average and 90,000 BTU/hr input with good. Improving insulation and air sealing is often cheaper than a bigger furnace.

How do I allow for high ceilings?

The BTU per sq ft figures assume 8 ft ceilings, so this tool scales the load by ceiling height ÷ 8. A 10 ft ceiling in the same 1,600 sq ft zone 5 home is a factor of 1.25, which takes the input from 100,000 BTU/hr input to 125,000 BTU/hr input. For a home with a mix of ceiling heights, use an average weighted by floor area.

Is it bad to buy a furnace that is too big?

Yes. An oversized furnace heats the house quickly, shuts off, and repeats, which is called short cycling. That wears the parts, wastes fuel, leaves temperature swings and uneven rooms, and costs more to buy. A furnace slightly too small is not a failure if it can run most of a cold day, which is why this tool adds no safety margin, but a result right between two sizes deserves a Manual J calculation. Two-stage and modulating furnaces cope with this better.

Which climate zone am I in?

The IECC and the US Department of Energy divide the United States into climate zones 1 to 7 (plus 8 for northern Alaska); a county-level map is on the DOE Building America and the Energy Code websites. Roughly: 1 and 2 are the Gulf Coast, Florida and the desert Southwest; 3 is the lower South and California coast; 4 is the mid-Atlantic and Pacific Northwest; 5 is the Midwest, Northeast and Mountain West; 6 and 7 are the northern tier. Zone 8 is not covered by this tool.

Why is the size bigger than my result, and what if it is above 120,000?

Furnaces come in steps, and this tool shows the smallest of 40,000, 60,000, 80,000, 100,000, 120,000 BTU/hr input that is at or above your input, never a smaller one: 100,000 BTU/hr goes up to 100,000. If the input is above 120,000 BTU/hr, the tool says so instead of picking a size: get a Manual J, and expect a larger unit or two systems. Those sizes are typical; some makers also sell 45,000, 70,000, 90,000 and 110,000. Take the exact input to your contractor and ask what they would choose.

Does this work for a heat pump, boiler or electric furnace?

The heat loss estimate applies to any heating system, but the furnace input step does not. It assumes a fuel-burning furnace with an AFUE. A heat pump is sized by its heating capacity at the design temperature and its efficiency drops in the cold; a boiler has its own AFUE; an electric furnace turns nearly all the electricity into heat, so its input equals its output in BTU/hr (3,412 BTU/hr per kW). Ask a contractor for a Manual J before choosing any of them.

Furnace Size Calculator: How Many BTU/hr Does My Home Need?

A 1,600 sq ft home in climate zone 5 with average insulation and 8 ft ceilings loses roughly 80,000 BTU/hr on a design day by the rule of thumb of 50 BTU/hr per sq ft, so the furnace needs about 80,000 BTU/hr of output. At 80% AFUE that is 100,000 BTU/hr input, so the smallest typical size that covers it is 100,000 BTU/hr; a 95% AFUE condensing furnace needs only 84,211 BTU/hr input. Choose your zone, insulation, ceiling height and furnace efficiency to adjust. This is a rule of thumb, not a Manual J load calculation; a contractor’s Manual J is how a furnace should actually be chosen.

Figures based on the sources listed belowUpdated 2026-10-15How we calculate

Heated length

Heated width

Climate zone (IECC / DOE)Find your zone on the DOE / IECC climate zone map. 1 is the hottest, 7 the coldest. The BTU per sq ft figures are typical HVAC-guide rules of thumb.

Insulation qualityPoor: older or drafty, little attic or wall insulation. Good: newer, well sealed and well insulated.

Ceiling height (ft)

The load is scaled by ceiling height ÷ 8 ft.

Furnace efficiency (AFUE)AFUE is the share of the fuel’s heat that ends up as heat in the home over a year. Standard efficiency is about 80%, condensing 90% to 98%.

100,000 BTU/hr input

Heat loss estimate: 80,000 BTU/hr (rule of thumb, zone 5: 1,600 sq ft × 50 BTU/hr per sq ft, adjusted)
Furnace output needed: 80,000 BTU/hr of heat delivered
Furnace input needed: 100,000 BTU/hr at 80% AFUE
Smallest common size that covers it: 100,000 BTU/hr input (typical sizes: 40,000 to 120,000)

Shopping list

Furnace filter
Programmable thermostat
Weatherstripping
Carbon monoxide detector
Tape measure (25 ft)

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BTU/hr per sq ft by IECC / DOE climate zone (typical, rule of thumb)

Typical heating load for a home with average insulation and 8 ft ceilings, and what it comes to for 1,600 sq ft. These are rule-of-thumb figures from HVAC guides, not DOE values, and not a Manual J result.
Climate zone	Examples	BTU/hr per sq ft	For 1,600 sq ft
Zone 1	southern Florida, Hawaii	30	48,000 BTU/hr
Zone 2	Gulf Coast, southern Texas, Phoenix	35	56,000 BTU/hr
Zone 3	Atlanta, Dallas, northern California coast	40	64,000 BTU/hr
Zone 4	Washington DC, St. Louis, Seattle	45	72,000 BTU/hr
Zone 5	Chicago, Boston, Denver	50	80,000 BTU/hr
Zone 6	Minneapolis, northern Michigan, Montana	55	88,000 BTU/hr
Zone 7	northern Minnesota, northern Maine	60	96,000 BTU/hr

Furnace input needed at 80% AFUE, with the common size that covers it

BTU/hr input worked by this calculator for average insulation and 8 ft ceilings at 80% AFUE; the smallest typical size at or above it in brackets.
Home size	Zone 2	Zone 4	Zone 5	Zone 7
1,000 sq ft (25 × 40 ft)	43,750 (60,000)	56,250 (60,000)	62,500 (80,000)	75,000 (80,000)
1,500 sq ft (30 × 50 ft)	65,625 (80,000)	84,375 (100,000)	93,750 (100,000)	112,500 (120,000)
1,600 sq ft (40 × 40 ft)	70,000 (80,000)	90,000 (100,000)	100,000 (100,000)	120,000 (120,000)
2,500 sq ft (50 × 50 ft)	109,375 (120,000)	140,625 (above 120,000)	156,250 (above 120,000)	187,500 (above 120,000)

What AFUE does to the input, for the 1,600 sq ft zone 5 example

The same heat output needs a smaller burner as AFUE rises: input = output ÷ AFUE.
AFUE	Type	Output needed	Input needed	Smallest typical size that covers it
80%	standard efficiency	80,000 BTU/hr	100,000 BTU/hr	100,000
90%	condensing	80,000 BTU/hr	88,889 BTU/hr	100,000
92%	condensing	80,000 BTU/hr	86,957 BTU/hr	100,000
95%	condensing	80,000 BTU/hr	84,211 BTU/hr	100,000
97%	condensing	80,000 BTU/hr	82,474 BTU/hr	100,000

Methodology & assumptions

This is a rule-of-thumb estimate, not a Manual J load calculation. Manual J, published by ACCA (the Air Conditioning Contractors of America), is the method contractors use to size a heating and cooling system: it counts wall, ceiling and floor insulation, window area and type, air leakage, ducts and the local design temperature. A square-foot rule sees none of that, and rule-of-thumb figures from different HVAC guides differ by 10 BTU/hr per sq ft or more. Steps. 1) Heated floor area = length × width, in sq ft (add up the floors if the home has more than one; leave out unheated basements, garages and attics). 2) Base load = area × BTU/hr per sq ft for the climate zone. The zones are the IECC / DOE climate zones, 1 (hottest) to 7 (coldest). The figures used here are zone 1: 30, zone 2: 35, zone 3: 40, zone 4: 45, zone 5: 50, zone 6: 55, zone 7: 60. They are typical values, the middle of the ranges HVAC manufacturers and guides print for warm (about 30 to 35), moderate (about 40 to 50) and cold (about 50 to 60) climates; no DOE or ENERGY STAR page publishes a zone-by-zone table, so the split into seven zones is this site’s interpolation and is marked as typical, not verified. 3) Insulation: poor adds 10% and good subtracts 10%, the adjustment several HVAC guides use; average changes nothing. The table assumes average insulation. 4) Ceiling height: the load is multiplied by ceiling height ÷ 8 ft, because the table assumes 8 ft ceilings and a taller ceiling holds more air to heat. For a home with a mix of heights, use the average. 5) Heat loss estimate (BTU/hr) = area × BTU/hr per sq ft × insulation factor × ceiling height ÷ 8. 6) Furnace output needed = the heat loss; this tool adds no extra safety margin, because an oversized furnace short-cycles, wears faster, costs more and heats less evenly. 7) Furnace input = output ÷ AFUE. AFUE, annual fuel utilization efficiency, is the US Department of Energy’s measure of how much of the fuel’s energy a furnace turns into heat for the home over a typical year, as a percentage: an 80% AFUE furnace sends 80 cents of every fuel dollar into the house and the rest out the flue. Standard-efficiency furnaces are about 80% AFUE; condensing furnaces, which cool the flue gas enough to capture more heat, are 90% to about 98%. A 95% AFUE furnace therefore needs a smaller burner than an 80% one for the same heat: input = output ÷ 0.95 against output ÷ 0.80. DOE’s AFUE does not include heat lost from the ducts, which can be large when they run through an attic or crawl space. 8) Smallest common size that covers it: gas furnaces are sold in input sizes such as 40,000, 60,000, 80,000, 100,000 and 120,000 BTU/hr; the tool shows the smallest one at or above your input, so the furnace is never smaller than the load. These sizes are typical, not a complete list: some makers sell 45,000, 70,000, 90,000 or 110,000. An input above 120,000 BTU/hr, the largest size listed, is shown as above the common single-furnace range rather than rounded down: it needs a Manual J load calculation and may call for a larger commercial unit or two systems. Rounding up can mean a furnace up to 20,000 BTU/hr larger than the load, so ask a contractor about a two-stage or modulating model. Many furnaces also have two stages or modulate, running at 65% to 70% of their rating most of the time. This tool does not size a heat pump, a boiler or electric heat, and it does not size ducts.

Bryant (Carrier), What Size Furnace Do I Need?: a manufacturer guide giving about 30 to 35 BTU/hr per sq ft for warm climates, 40 to 50 for moderate and 50 to 60 for cold, with example sizes for 1,000 to 2,500 sq ft homes, a note that insulation, ceiling height and air leakage shift the need, and the advice to get a professional load calculation. The seven zone figures on this page are this site’s interpolation of such ranges
PickHVAC, Heating BTU Calculator: an HVAC guide with a 1 to 7 climate zone table of BTU per sq ft, from about 30 to 35 in zones 1 and 2 to about 60 in zone 7, and a general 30 to 60 range. Checked against a search result summary only; the page itself could not be opened from this site, and its zone figures differ from other guides by one zone
Inch Calculator, Furnace BTU Calculator: a zone table of 30 to 35 (zone 1) to 50 to 60 (zone 5) BTU per sq ft for average insulation, with about 10% more for poor insulation and about 10% less for very good. A secondary, non-government source, checked from a search result summary only
US Department of Energy, Energy Saver, Furnaces and Boilers: AFUE as the share of the fuel’s energy converted to heat for the home over a typical year, with mid-efficiency furnaces at 80% to 83% AFUE, and the note that AFUE excludes duct losses, which can reach 35% of the furnace’s output when ducts run through unconditioned space
US Department of Energy, Energy Conservation Standards for Consumer Furnaces (final rule, December 2023): sets a 95% AFUE minimum for new non-weatherized gas furnaces, with compliance expected in late 2028; the current minimum is 80%, and 95% AFUE requires condensing technology (as reported by Rheem’s industry alert and the Columbia Sabin Center)
ACCA, Manual J, Residential Load Calculation: the standard method for sizing home heating and cooling equipment, which this page’s rule of thumb is not
Unverified: the per-zone BTU/hr per sq ft figures (30 to 60) and the plus or minus 10% insulation adjustment are typical figures from HVAC guides, not published by DOE or ENERGY STAR. The 40,000 to 120,000 BTU/hr list of common furnace input sizes is typical of gas furnace product lines, not a standard

How to estimate the furnace size for a home

Measure the heated floor area. Multiply the length and width of the heated living space in feet to get square feet. Add the floor areas if there is more than one story; leave out unheated space.
Pick your climate zone. Find your IECC / DOE climate zone, 1 (hot) to 7 (cold), and use its typical BTU/hr per sq ft: about 30 in zone 1 up to about 60 in zone 7.
Adjust for insulation and ceiling height. Add 10% for poor insulation or subtract 10% for good insulation, then multiply by the ceiling height divided by 8 ft.
Divide by the furnace AFUE. The result is the heat the furnace must deliver (output). Divide it by the AFUE as a decimal, for example 0.80 or 0.95, to get the input in BTU/hr.
Choose a common size and confirm it. Pick the smallest typical input size at or above the result (40,000, 60,000, 80,000, 100,000 or 120,000 BTU/hr); above 120,000, get a Manual J and expect a larger unit or two systems. Ask a contractor to confirm it with a Manual J load calculation.

Worked examples

Project	Result
1,600 sq ft, zone 5, average insulation, 8 ft ceilings, 80% AFUE	80,000 BTU/hr heat loss, 100,000 BTU/hr input, 100,000 BTU/hr size covers it
The same home with a 95% AFUE condensing furnace	84,211 BTU/hr input for the same 80,000 BTU/hr of output, 100,000 BTU/hr size covers it
The same home with poor insulation (+10%)	88,000 BTU/hr heat loss, 110,000 BTU/hr input, 120,000 BTU/hr size covers it
Well-insulated version with a 95% AFUE furnace (−10%)	72,000 BTU/hr heat loss, 75,789 BTU/hr input, 80,000 BTU/hr size covers it
The same home with 10 ft ceilings (× 1.25)	100,000 BTU/hr heat loss, 125,000 BTU/hr input, above the common single-furnace range (get a Manual J)
1,200 sq ft home in zone 2 (35 BTU/hr per sq ft), 80% AFUE	42,000 BTU/hr heat loss, 52,500 BTU/hr input, 60,000 BTU/hr size covers it
1,500 sq ft home in zone 7, poor insulation, 80% AFUE	99,000 BTU/hr heat loss, 123,750 BTU/hr input, above the common single-furnace range (get a Manual J)

Frequently asked questions

What size furnace do I need for my house?: Multiply the heated floor area by the BTU/hr per sq ft for your climate zone, adjust for insulation and ceiling height, then divide by the furnace’s AFUE. For 1,600 sq ft in zone 5 with average insulation that is 80,000 BTU/hr of heat, or 100,000 BTU/hr input at 80% AFUE, so the smallest typical size that covers it is 100,000 BTU/hr. It is a rule of thumb: a Manual J load calculation is the proper way to choose.
What is the difference between furnace input and output?: Input is the fuel the burner uses, in BTU/hr; output is the heat that reaches the home. Output = input × AFUE. A furnace labelled 100,000 BTU/hr input at 80% AFUE delivers 80,000 BTU/hr, and at 95% AFUE it delivers 95,000. Furnaces are named and sold by input, so the size you shop for is the output you need divided by AFUE: 80,000 BTU/hr of output is 100,000 BTU/hr input at 80% and 84,211 BTU/hr input at 95%.
What is AFUE?: AFUE, annual fuel utilization efficiency, is the US Department of Energy’s measure of the share of a furnace’s fuel energy that becomes heat for the home over a typical year. An 80% AFUE furnace turns 80% of the fuel into house heat and loses the rest up the flue; a 95% condensing furnace loses only 5%. DOE’s AFUE leaves out heat lost from ducts, which can be large in an attic or crawl space. The federal minimum for new gas furnaces is currently 80%, and DOE has set 95% for new non-weatherized gas furnaces from late 2028.
Is this a Manual J calculation?: No. Manual J, published by ACCA, is the method contractors use to size heating and cooling equipment. It counts insulation levels, window area and type, air leakage, ducts and the local design temperature for each room. This page uses one BTU per sq ft figure for a climate zone, so it can be off by 25% or more for an unusual house. Use it for a ballpark when budgeting or checking a quote, and ask your contractor for a Manual J.
How does insulation change the size?: This tool adds 10% for poor insulation and subtracts 10% for good insulation, the adjustment several HVAC guides use. For the 1,600 sq ft zone 5 home at 80% AFUE that is 110,000 BTU/hr input with poor insulation, 100,000 BTU/hr input with average and 90,000 BTU/hr input with good. Improving insulation and air sealing is often cheaper than a bigger furnace.
How do I allow for high ceilings?: The BTU per sq ft figures assume 8 ft ceilings, so this tool scales the load by ceiling height ÷ 8. A 10 ft ceiling in the same 1,600 sq ft zone 5 home is a factor of 1.25, which takes the input from 100,000 BTU/hr input to 125,000 BTU/hr input. For a home with a mix of ceiling heights, use an average weighted by floor area.
Is it bad to buy a furnace that is too big?: Yes. An oversized furnace heats the house quickly, shuts off, and repeats, which is called short cycling. That wears the parts, wastes fuel, leaves temperature swings and uneven rooms, and costs more to buy. A furnace slightly too small is not a failure if it can run most of a cold day, which is why this tool adds no safety margin, but a result right between two sizes deserves a Manual J calculation. Two-stage and modulating furnaces cope with this better.
Which climate zone am I in?: The IECC and the US Department of Energy divide the United States into climate zones 1 to 7 (plus 8 for northern Alaska); a county-level map is on the DOE Building America and the Energy Code websites. Roughly: 1 and 2 are the Gulf Coast, Florida and the desert Southwest; 3 is the lower South and California coast; 4 is the mid-Atlantic and Pacific Northwest; 5 is the Midwest, Northeast and Mountain West; 6 and 7 are the northern tier. Zone 8 is not covered by this tool.
Why is the size bigger than my result, and what if it is above 120,000?: Furnaces come in steps, and this tool shows the smallest of 40,000, 60,000, 80,000, 100,000, 120,000 BTU/hr input that is at or above your input, never a smaller one: 100,000 BTU/hr goes up to 100,000. If the input is above 120,000 BTU/hr, the tool says so instead of picking a size: get a Manual J, and expect a larger unit or two systems. Those sizes are typical; some makers also sell 45,000, 70,000, 90,000 and 110,000. Take the exact input to your contractor and ask what they would choose.
Does this work for a heat pump, boiler or electric furnace?: The heat loss estimate applies to any heating system, but the furnace input step does not. It assumes a fuel-burning furnace with an AFUE. A heat pump is sized by its heating capacity at the design temperature and its efficiency drops in the cold; a boiler has its own AFUE; an electric furnace turns nearly all the electricity into heat, so its input equals its output in BTU/hr (3,412 BTU/hr per kW). Ask a contractor for a Manual J before choosing any of them.