AC BTU Calculator

This free AC BTU calculator tells you what size air conditioner you need for a room. Enter the room size in feet or metres and it gives the cooling capacity in BTU per hour, tons and kW, adjusted for sun, the number of people, kitchens and high ceilings, and suggests the nearest standard window, portable or mini-split AC size — plus roughly how many watts it will draw.

How AC size is calculated

Base BTU = ENERGY STAR chart value for the room's area
× ceiling height ÷ 8 ft (only for ceilings above 8 ft)
× 0.9 if heavily shaded, × 1.1 if very sunny
+ 600 BTU per person over two, + 4,000 BTU for a kitchen

Worked example

A 15 × 20 ft living room is 300 sq ft, which the chart puts at 7,000 BTU. If it is very sunny, add 10%: 7,700 BTU. With a 10 ft ceiling instead of 8 ft, multiply by 10 ÷ 8 again: 9,625 BTU — so a 10,000 BTU unit.

AC size chart by room size (ENERGY STAR)

Room area	Area in m²	Cooling capacity
100–150 sq ft	9–14 m²	5,000 BTU
150–250 sq ft	14–23 m²	6,000 BTU
250–300 sq ft	23–28 m²	7,000 BTU
300–350 sq ft	28–33 m²	8,000 BTU
350–400 sq ft	33–37 m²	9,000 BTU
400–450 sq ft	37–42 m²	10,000 BTU
450–550 sq ft	42–51 m²	12,000 BTU
550–700 sq ft	51–65 m²	14,000 BTU
700–1,000 sq ft	65–93 m²	18,000 BTU
1,000–1,200 sq ft	93–111 m²	21,000 BTU
1,200–1,400 sq ft	111–130 m²	23,000 BTU
1,400–1,500 sq ft	130–139 m²	24,000 BTU
1,500–2,000 sq ft	139–186 m²	30,000 BTU
2,000–2,500 sq ft	186–232 m²	34,000 BTU

What size AC for common rooms?

Normal sun, two people and 8 ft ceilings.

Room	Area	BTU needed	Buy a
10 × 10 ft bedroom	100 sq ft	5,000	5,000 BTU
12 × 12 ft bedroom	144 sq ft	5,000	5,000 BTU
12 × 15 ft room	180 sq ft	6,000	6,000 BTU
15 × 20 ft living room	300 sq ft	7,000	8,000 BTU
20 × 20 ft room / garage bay	400 sq ft	9,000	10,000 BTU
20 × 25 ft open-plan room	500 sq ft	12,000	12,000 BTU
25 × 40 ft (1,000 sq ft)	1,000 sq ft	18,000	18,000 BTU

BTU to tons, kW and watts

AC size	Tons	Cooling kW	Typical power draw
5,000 BTU	0.42	1.5 kW	≈ 455 W
8,000 BTU	0.67	2.3 kW	≈ 727 W
10,000 BTU	0.83	2.9 kW	≈ 909 W
12,000 BTU	1	3.5 kW	≈ 1,091 W
18,000 BTU	1.5	5.3 kW	≈ 1,636 W
24,000 BTU	2	7 kW	≈ 2,182 W
36,000 BTU	3	10.6 kW	≈ 3,273 W

Power draw assumes an EER of 11; check the yellow EnergyGuide label or spec sheet for your model. Whole-house central air systems should be sized with a professional Manual J load calculation.

Related calculations

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Frequently asked questions

How many BTU do I need for a 12×12 room?

A 12 × 12 ft room is 144 sq ft, which needs about 5,000 BTU according to the ENERGY STAR sizing chart. Add 10% if the room gets a lot of sun, and 600 BTU for each person beyond two who regularly uses the room.

How many BTU do I need for 500 square feet?

About 12,000 BTU (1 ton) for a typical room of 450–550 sq ft. A sunny room needs about 13,200 BTU, and a kitchen adds 4,000 BTU.

How many BTU per square foot do I need?

A common rule of thumb is about 20 BTU per square foot for rooms with 8 ft ceilings in a typical climate. The ENERGY STAR chart this calculator uses is a little more precise and needs fewer BTU per square foot for larger spaces. Hot climates, poor insulation and big west-facing windows push the figure up.

Is a bigger air conditioner better?

No. An oversized unit cools the room quickly and switches off before it has removed enough moisture, leaving the room cold and clammy. It also cycles on and off more, which wastes energy and wears it out. Choose the size that matches the room, rounded up to the next available model.

How many watts does a 12,000 BTU air conditioner use?

About 1,091 W at a typical efficiency (EER of 11), and less when the thermostat cycles it off. Inverter models use noticeably less at part load. To see what that costs to run, enter the watts in the electricity cost calculator.

How do I convert BTU to tons and kW?

One ton of cooling is 12,000 BTU per hour, and 1 BTU/h is about 0.293 W. So a 24,000 BTU unit is 2 tons, or about 7 kW of cooling. The kW of cooling is not the electrical power it draws — that is roughly the cooling divided by 3.