What size air conditioner do I need for my room?

Multiply the room’s length by its width, find that area in the ENERGY STAR chart for its BTU/hr, then adjust for sun, people and a kitchen. A 12 × 14 ft room is 168 sq ft, in the 150 to 250 sq ft row, so 6,000 BTU/hr. If that room is very sunny the figure goes up 10% to 6,600 BTU/hr, and as a kitchen it adds 4,000: 10,000 BTU/hr.

What is a BTU, and how many BTUs make a ton?

A BTU (British thermal unit) is a unit of heat; an air conditioner’s rating is how many BTUs of heat it removes each hour, written BTU/hr. A ton of cooling is 12,000 BTU/hr, so 6,000 BTU/hr is 0.5 tons. The word comes from the heat needed to melt a ton of ice in a day; it has nothing to do with the weight of the unit.

Is this the same as a Manual J load calculation?

No. This is the room air conditioner rule of thumb from ENERGY STAR: it looks at the size of one room and a few conditions. A Manual J load calculation is what contractors use to size central and whole-house systems; it counts the insulation, the windows and which way they face, air leakage, the local climate and the heat from people and appliances, and it is done for the whole building or each zone. Use this page to choose a window or portable unit for a single room, and a Manual J calculation or a contractor for a ducted or multi-room system.

What if my room falls between two sizes on the chart?

The chart’s ranges share their end points, so a room of exactly 150 sq ft is read in the lower row (5,000 BTU/hr) by this tool, and one of 151 sq ft in the next (6,000). Most rooms fall inside a row. When a result sits between two sizes of unit on sale, go to the nearer one, and up rather than down if the room is sunny or often full, but do not jump several sizes: ENERGY STAR warns that bigger is not better. A 15 × 10 ft room that is heavily shaded comes to 4,500 BTU/hr.

How do I add extra people and a kitchen?

The chart assumes two people. ENERGY STAR adds 600 BTU/hr for each person beyond two who regularly use the room: a 20 × 25 ft room (12,000 BTU/hr from the chart) with four people adds 1,200, for 13,200 BTU/hr. A kitchen adds 4,000 BTU/hr for the heat from cooking. A 12 × 20 ft kitchen-diner that is very sunny, with five people, comes to 12,400 BTU/hr.

How do I size for an open archway or alcove?

Enter the floor area of the extra space in “Extra area to cool”, and it is added to length × width. A 14 × 16 ft room with a 60 sq ft alcove is 284 sq ft: 7,000 BTU/hr. A room that opens wide onto a large space can be more than one room unit will cool; in that case treat the two as one area, or look at a ducted or multi-zone system.

What if my space is bigger than the chart?

The ENERGY STAR room air conditioner chart ends at 1,400 sq ft (23,000 BTU/hr here), and this tool will not extend it. Above that it shows “at least 23,000 BTU/hr (beyond the room AC chart)” plus a warning: that figure is a minimum, not a size. A space that large is better served by a central or multi-unit system sized with a Manual J load calculation by an HVAC professional.

Do ceiling height, windows and insulation matter?

Yes, and this chart does not count them. It is calculated for an 8 ft ceiling, and ENERGY STAR notes that taller ceilings may need more capacity without giving a figure. Large or west-facing windows, a top-floor room under a roof, poor insulation and a hot climate also raise the load. The sun setting is the only one of these the chart adjusts for. If several apply strongly, choose the sunny setting, or use a Manual J calculation.

Is it bad to buy an air conditioner that is too big?

ENERGY STAR advises against it: bigger is not always better, and a unit larger than the room needs costs more to buy and to run. Follow the chart rather than rounding up by a size or two. A unit that is too small is a problem as well, since it runs all day and may not catch up on the hottest days.

How does this compare with the 20 BTU per square foot rule?

The US Department of Energy gives a rough rule of about 20 BTU/hr per sq ft for room units. The ENERGY STAR chart is not a straight line: for the 168 sq ft room above it gives 6,000 BTU/hr (about 36 per sq ft) where 20 per sq ft would give 3,360. The chart is what this calculator uses; the 20 BTU rule is only a quick cross-check.

Air Conditioner BTU Calculator: What Size Room AC Do I Need?

A 12 × 14 ft room is 168 sq ft, which falls in the ENERGY STAR chart’s 150 to 250 sq ft row: 6,000 BTU/hr, or 0.5 tons (12,000 BTU/hr is 1 ton), for average sun, two people and no kitchen. Enter the room size, how sunny it is, how many people use it and whether it is a kitchen to adjust that. This is the room air conditioner rule of thumb from ENERGY STAR, not a Manual J load calculation, which is how whole-house and central systems are sized.

Figures based on the sources listed belowUpdated 2026-10-08How we calculate

Room length

Room width

Extra area to cool (optional) (sq ft)

Floor area of an open alcove, dining nook or archway the unit also has to cool, added to length × width. Leave 0 for a closed room.

Sun exposureENERGY STAR: reduce the capacity by 10% for a heavily shaded room and increase it by 10% for a very sunny one.

People who regularly use the room

The chart covers two people. ENERGY STAR adds 600 BTU/hr for each person beyond two.

Is it a kitchen?ENERGY STAR: add 4,000 BTU/hr if the unit is for a kitchen, for the heat of cooking.

6,000 BTU/hr

ENERGY STAR chart band: 150 to 250 sq ft, 6,000 BTU/hr (your area: 168 sq ft)
Adjustments: none (average sun, two people or fewer, not a kitchen)
Tons of cooling: 0.5 tons (12,000 BTU/hr = 1 ton)

Shopping list

Window air conditioner
Window AC insulation seal kit
Window AC support bracket
Blackout curtains
Tape measure (25 ft)

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ENERGY STAR room air conditioner sizing chart

Area to be cooled against the capacity needed, for an 8 ft ceiling and two people. A room exactly on a shared end point (150, 250, 300 sq ft and so on) is put in the lower row. The last column says where each row was checked; the two rows above 1,000 sq ft are unverified.
Area to cool	Capacity (BTU/hr)	Tons	Checked against
100 to 150 sq ft	5,000	0.42 tons	ENERGY STAR text
150 to 250 sq ft	6,000	0.5 tons	ENERGY STAR text
250 to 300 sq ft	7,000	0.58 tons	ENERGY STAR text
300 to 350 sq ft	8,000	0.67 tons	ENERGY STAR text
350 to 400 sq ft	9,000	0.75 tons	ENERGY STAR text
400 to 450 sq ft	10,000	0.83 tons	ENERGY STAR text
450 to 550 sq ft	12,000	1 ton	ENERGY STAR text
550 to 700 sq ft	14,000	1.17 tons	ENERGY STAR text
700 to 1,000 sq ft	18,000	1.5 tons	ENERGY STAR text
1,000 to 1,200 sq ft	21,000	1.75 tons	retailer copies of the chart (unverified)
1,200 to 1,400 sq ft	23,000	1.92 tons	retailer copies of the chart (unverified)

Room AC size for common rooms, with sun and kitchen adjustments

BTU/hr, worked by this calculator for two people. The shaded and sunny columns apply −10% and +10% to the chart value; the kitchen column adds 4,000 BTU/hr to the room at average sun.
Room	Sq ft	Average sun	Heavily shaded	Very sunny	Used as a kitchen
Small bedroom, 10 × 10 ft	100	5,000	4,500	5,500	9,000
Bedroom, 12 × 14 ft	168	6,000	5,400	6,600	10,000
Living room, 15 × 20 ft	300	7,000	6,300	7,700	11,000
Large living room, 20 × 25 ft	500	12,000	10,800	13,200	16,000

Adjustments to the chart value

The ENERGY STAR adjustments this calculator applies, in the order it applies them.
Condition	Adjustment
Heavily shaded room	−10% of the chart value
Very sunny room	+10% of the chart value
More than two people regularly in the room	+600 BTU/hr for each extra person
Kitchen	+4,000 BTU/hr

Methodology & assumptions

This is the sizing method ENERGY STAR publishes for room air conditioners (window, through-the-wall and portable units), in four steps. 1) Area = length × width, plus any extra open area you enter, in sq ft. 2) Base capacity = the row of the ENERGY STAR chart that holds the area, from 5,000 BTU/hr for 100 to 150 sq ft to 18,000 BTU/hr for 700 to 1,000 sq ft (the full chart is in the table below). The published ranges share their end points (100 to 150, then 150 to 250), so a room of exactly 150 sq ft is put in the lower row. Below 100 sq ft the chart has no row, and this tool uses the smallest one, 5,000 BTU/hr. Above 1,400 sq ft the chart ends: the result is then shown as “at least 23,000 BTU/hr (beyond the room AC chart)”, a minimum and not a size, with a note to get a Manual J load calculation, because no source was found for larger rows. 3) Sun: reduce by 10% for a heavily shaded room, increase by 10% for a very sunny one. 4) Add 600 BTU/hr for each person beyond two who regularly use the room, and 4,000 BTU/hr for a kitchen. ENERGY STAR lists these adjustments without saying in what order to apply them; this tool takes the percentage of the chart value first and then adds the people and the kitchen, which are fixed amounts. Tons = BTU/hr ÷ 12,000. The chart is calculated for an 8 ft ceiling; ENERGY STAR notes that higher ceilings may call for more capacity, but gives no figure, so none is applied here. The answer is a capacity to look for, not a model: room units come in a limited set of sizes, so choose the size nearest to it, going up if you are between two. Bigger is not better: ENERGY STAR warns against oversizing, since a unit larger than the room costs more to buy and to run. What this is not: a Manual J load calculation. Manual J (published by ACCA, the Air Conditioning Contractors of America) is the method used to size central and whole-house systems; it counts insulation, window area and orientation, air leakage, climate and the heat given off inside the home, none of which a room-size chart sees. Use a Manual J calculation, or a contractor, for ducted or multi-room systems, and treat this page as a starting point for a single room. The two rows above 1,000 sq ft (21,000 and 23,000 BTU/hr) are from retailer copies of the chart and could not be checked against ENERGY STAR text; they are marked as such in the table. The 20 BTU/hr per sq ft rule of thumb from the US Department of Energy is a cross-check only: the chart gives 6,000 BTU/hr for 168 sq ft, where 20 BTU/hr per sq ft would give 3,360.

ENERGY STAR, Room Air Conditioners (marketing flyer ROOM_AIRCONDITIONER.pdf): the sizing chart of area to cool against capacity needed in BTU/hr, and the adjustments: reduce capacity by 10% for a heavily shaded room, increase it by 10% for a very sunny one, add 600 BTU/hr for each person beyond two, add 4,000 BTU/hr for a kitchen. This older flyer prints the 14,000 BTU/hr row as 500 to 700 sq ft
ENERGY STAR, How To Choose the Right Sized Window AC (Ask the Experts) and the Room Air Conditioners page: the chart as currently published (450 to 550 sq ft at 12,000 BTU/hr, 550 to 700 sq ft at 14,000), calculated for an 8 ft ceiling, with the warning that bigger is not always better
ENERGY STAR, Room Air Conditioners fact sheet (ES_RoomAC_FactSheet_23.pdf): the sun adjustments and the kitchen allowance, as a sizing guide
US Department of Energy, Room Air Conditioners: room units generally range from about 5,500 to 14,000 BTU/hr, with a rough rule of about 20 BTU/hr per sq ft and the advice to allow for room height, climate, shading and window size
Unverified: the 1,000 to 1,200 sq ft (21,000 BTU/hr) and 1,200 to 1,400 sq ft (23,000 BTU/hr) rows come from retailer copies of the chart (for example removeandreplace.com), not from an ENERGY STAR page this site could read; the rows to 1,000 sq ft were checked against ENERGY STAR text. A ton of cooling is 12,000 BTU/hr by definition

How to size a room air conditioner

Measure the room. Measure the length and width of the room in feet and multiply to get square feet. Add the floor area of any open alcove or archway the unit also has to cool.
Find the chart value. Look the area up in the ENERGY STAR room air conditioner chart: for example 100 to 150 sq ft is 5,000 BTU/hr and 150 to 250 sq ft is 6,000 BTU/hr.
Adjust for sun. Reduce the value by 10% if the room is heavily shaded, or increase it by 10% if it is very sunny.
Add people and the kitchen. Add 600 BTU/hr for each person beyond two who regularly use the room, and 4,000 BTU/hr if the unit is for a kitchen.
Choose a unit. Pick the unit whose BTU/hr rating is nearest the result, divide by 12,000 for tons, and do not go far above it: bigger is not better.

Worked examples

Project	Result
12 × 14 ft bedroom, average sun, two people	6,000 BTU/hr, 0.5 tons (168 sq ft, 150 to 250 sq ft row)
The same room, very sunny	6,600 BTU/hr, 0.55 tons (6,000 + 10%)
The same size as a kitchen	10,000 BTU/hr, 0.83 tons (6,000 + 4,000)
15 × 10 ft room, heavily shaded (exactly 150 sq ft, read in the lower row)	4,500 BTU/hr, 0.38 tons (5,000 − 10%)
20 × 25 ft living room with four people	13,200 BTU/hr, 1.1 tons (12,000 + 2 × 600)
14 × 16 ft room with a 60 sq ft alcove	7,000 BTU/hr, 0.58 tons (284 sq ft)
12 × 20 ft sunny kitchen-diner with five people	12,400 BTU/hr, 1.03 tons (240 sq ft, 6,000 + 10% + 3 × 600 + 4,000)

Frequently asked questions

What size air conditioner do I need for my room?: Multiply the room’s length by its width, find that area in the ENERGY STAR chart for its BTU/hr, then adjust for sun, people and a kitchen. A 12 × 14 ft room is 168 sq ft, in the 150 to 250 sq ft row, so 6,000 BTU/hr. If that room is very sunny the figure goes up 10% to 6,600 BTU/hr, and as a kitchen it adds 4,000: 10,000 BTU/hr.
What is a BTU, and how many BTUs make a ton?: A BTU (British thermal unit) is a unit of heat; an air conditioner’s rating is how many BTUs of heat it removes each hour, written BTU/hr. A ton of cooling is 12,000 BTU/hr, so 6,000 BTU/hr is 0.5 tons. The word comes from the heat needed to melt a ton of ice in a day; it has nothing to do with the weight of the unit.
Is this the same as a Manual J load calculation?: No. This is the room air conditioner rule of thumb from ENERGY STAR: it looks at the size of one room and a few conditions. A Manual J load calculation is what contractors use to size central and whole-house systems; it counts the insulation, the windows and which way they face, air leakage, the local climate and the heat from people and appliances, and it is done for the whole building or each zone. Use this page to choose a window or portable unit for a single room, and a Manual J calculation or a contractor for a ducted or multi-room system.
What if my room falls between two sizes on the chart?: The chart’s ranges share their end points, so a room of exactly 150 sq ft is read in the lower row (5,000 BTU/hr) by this tool, and one of 151 sq ft in the next (6,000). Most rooms fall inside a row. When a result sits between two sizes of unit on sale, go to the nearer one, and up rather than down if the room is sunny or often full, but do not jump several sizes: ENERGY STAR warns that bigger is not better. A 15 × 10 ft room that is heavily shaded comes to 4,500 BTU/hr.
How do I add extra people and a kitchen?: The chart assumes two people. ENERGY STAR adds 600 BTU/hr for each person beyond two who regularly use the room: a 20 × 25 ft room (12,000 BTU/hr from the chart) with four people adds 1,200, for 13,200 BTU/hr. A kitchen adds 4,000 BTU/hr for the heat from cooking. A 12 × 20 ft kitchen-diner that is very sunny, with five people, comes to 12,400 BTU/hr.
How do I size for an open archway or alcove?: Enter the floor area of the extra space in “Extra area to cool”, and it is added to length × width. A 14 × 16 ft room with a 60 sq ft alcove is 284 sq ft: 7,000 BTU/hr. A room that opens wide onto a large space can be more than one room unit will cool; in that case treat the two as one area, or look at a ducted or multi-zone system.
What if my space is bigger than the chart?: The ENERGY STAR room air conditioner chart ends at 1,400 sq ft (23,000 BTU/hr here), and this tool will not extend it. Above that it shows “at least 23,000 BTU/hr (beyond the room AC chart)” plus a warning: that figure is a minimum, not a size. A space that large is better served by a central or multi-unit system sized with a Manual J load calculation by an HVAC professional.
Do ceiling height, windows and insulation matter?: Yes, and this chart does not count them. It is calculated for an 8 ft ceiling, and ENERGY STAR notes that taller ceilings may need more capacity without giving a figure. Large or west-facing windows, a top-floor room under a roof, poor insulation and a hot climate also raise the load. The sun setting is the only one of these the chart adjusts for. If several apply strongly, choose the sunny setting, or use a Manual J calculation.
Is it bad to buy an air conditioner that is too big?: ENERGY STAR advises against it: bigger is not always better, and a unit larger than the room needs costs more to buy and to run. Follow the chart rather than rounding up by a size or two. A unit that is too small is a problem as well, since it runs all day and may not catch up on the hottest days.
How does this compare with the 20 BTU per square foot rule?: The US Department of Energy gives a rough rule of about 20 BTU/hr per sq ft for room units. The ENERGY STAR chart is not a straight line: for the 168 sq ft room above it gives 6,000 BTU/hr (about 36 per sq ft) where 20 per sq ft would give 3,360. The chart is what this calculator uses; the 20 BTU rule is only a quick cross-check.

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