AC Size Calculator (BTU)
How many BTU an air conditioner needs for a room, adjusted for sun, people and kitchens.
Good to know. An oversized air conditioner cools fast, switches off, and leaves the room cold and clammy.
Approximate. Real-world values vary.
How it works
In plain words
Look up the room’s area on the ENERGY STAR chart, from 5,000 BTU/h for up to 150 ft² to 34,000 for 2,500 ft². Take off 10% for a heavily shaded room, add 10% for a very sunny one, add 600 BTU/h for each regular occupant over two, and 4,000 for a kitchen.
The formula
capacity = chart value × (0.9 shaded, 1.1 sunny) + 600 × (people − 2) + 4,000 if a kitchen 1 kW = 3,412 BTU/h · 1 ton = 12,000 BTU/h
Worked example
30 m² is 323 ft², which falls in the 300–350 ft² row of the chart: 8,000 BTU/h, about 2.34 kW.
Sources
- ENERGY STAR, Room Air Conditioners: sizing The capacity chart by room area and the adjustments for shade, sun, occupants and kitchens.
Sources checked September 30, 2026. 3 automated tests check its answers on every change.
About the AC size calculator (BTU)
An air conditioner has to be the right size for the room. Too small and it runs flat out without keeping up on hot days. Too big and it cools the room quickly, then switches off before it has taken the moisture out of the air, leaving the room cold and clammy while costing more to buy.
Cooling capacity is measured in BTU per hour in the US and in kilowatts elsewhere; 1 kW is about 3,412 BTU/h. This calculator uses the sizing chart ENERGY STAR publishes for room air conditioners, based on floor area, and applies its adjustments: 10% less for a heavily shaded room, 10% more for a very sunny one, 600 BTU/h more for each person over two who regularly uses the room, and 4,000 more for a kitchen.
The chart assumes normal ceilings of about 2.4 m (8 ft) and reasonable insulation. High ceilings, big windows, a top floor under the roof or a very hot climate all call for more. If a room falls between two sizes, the larger is usually the safer choice for a sunny room and the smaller for a shaded one.
The same sizes are a good starting point for a mini-split heat pump. For a whole house or central system, a professional load calculation (Manual J in the US) takes the building itself into account.
Questions people ask
What size air conditioner do I need?
About 20 BTU/h per square foot as a rough guide: 8,000 BTU/h for a 300–350 ft² (28–33 m²) room, adjusted for sun, people and kitchens.
How many BTU per square metre?
Roughly 200 to 250 BTU/h per square metre for normal rooms, depending on size, sun and use.
How many kW is 12,000 BTU?
About 3.5 kW, which is also 1 ton of cooling.