How to use it
- Measure the length and width of the room, and type the floor area. Choose the units first if you use feet.
- Enter the Ceiling height. A ceiling above about 8 feet needs a heat-load calculation.
- Choose Sun on the room and type the People who use the room. Tick The room is a kitchen if it is one.
- Read Cooling capacity needed and Nearest standard unit, then check How the figure is worked out.
How the capacity is worked out
The floor area in square feet is looked up in a common room-size sizing table. The guide that publishes it attributes the table to ENERGY STAR, the US government's efficiency programme, and assumes an 8-foot ceiling. Each band gives a capacity: 5,000 BTU/h up to 150 ft², 6,000 up to 250 ft², and so on up to 30,000 BTU/h for 2,000 ft².
The table is then corrected in the same guide: a very sunny room adds 10 %, more than two people add 10 % (the guide gives 10 to 20 %), and a kitchen adds 4,000 BTU/h. The total is rounded up to the next 100 BTU/h, and the nearest standard size is the smallest of 9,000, 12,000, 18,000 and 24,000 BTU/h that is at least as large.
base = table value for the floor area in ft²
total = base + 10 % if very sunny + 10 % if more than two people + 4,000 if a kitchen
kW = total ÷ 3,412.14 (1 kW = 3,412.14 BTU/h)What this calculator does not include
- Walls, windows, the roof and insulation, which decide most of a real room's heat gain. A heat-load calculation counts them one by one.
- The climate of your area: a room in a hot region needs more cooling than the same room in a mild one.
- Ceiling heights above 8 feet, which the table does not cover, and open-plan rooms joined to other rooms.
Worked example: a 20 m² room, very sunny, four people and a kitchen
20 m² is 215.3 ft², so the table gives 6,000 BTU/h. A very sunny room adds 10 % (600), more than two people add another 10 % (600), and the kitchen adds 4,000.
The total is 6,000 + 600 + 600 + 4,000 = 11,200 BTU/h, which is 3.28 kW. The nearest standard unit is 12,000 BTU/h.
| Plain room, two people | This example | |
|---|---|---|
| Capacity needed | 6,000 BTU/h | 11,200 BTU/h |
| In kilowatts | 1.76 kW | 3.28 kW |
| Standard unit | 9,000 BTU/h | 12,000 BTU/h |
Frequently asked questions
How many BTU do I need for a 20 m² room?
A 20 m² room, normal sun, two people and no kitchen needs about 6,000 BTU/h from the table, so a 9,000 BTU/h unit is the nearest standard size. A sunny kitchen for four people needs about 11,200 BTU/h.
What is a BTU?
A BTU, the British thermal unit, is the heat needed to raise one pound of water by one degree Fahrenheit. An air conditioner's capacity is given in BTU per hour; one kilowatt is about 3,412 BTU per hour.
Is a bigger air conditioner better?
No. An oversized unit cools the room quickly and switches off too soon, so it does not remove the humidity well and uses more energy at the start of each run. Choose the nearest standard size at or above the capacity.
Why does the table stop at 2,000 square feet?
Large rooms and open-plan spaces depend on the walls, windows and the number of openings, which a room-size table cannot show. A heat-load calculation by an installer is needed for them.
Does the tool store what I enter?
No. The calculation runs in your browser and nothing you type is sent to us or stored.
Sources
- Common room-size sizing guide (inchcalculator.com): the table and the sun, people and kitchen corrections; it attributes the table to ENERGY STAR, and the energystar.gov page could not be read
- Wikipedia: British thermal unit (1 W is about 3.412 BTU/h, so 1 kW is about 3,412 BTU/h)
Figures and text last checked on Tuesday, November 10, 2026.