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About the AC size calculator
An air conditioner that is too small runs all day and never cools the room. One that is too big cools fast, switches off early, and leaves the air damp and cold. The right size matters for comfort and for your electricity bill.
Type the room area and the ceiling height. Say how much sun the room gets and how many people use it. Open More options for the climate, the insulation, whether the room is a kitchen, or to type the length and width of the room.
You get the cooling power you need in BTU/h and in kW, the standard unit to buy, and the size in tons.
How to use it
- Choose m, m² or ft, sq ft
- Type the room area — or length and width under More options
- Type the ceiling height
- Choose the sun on the room and the number of people
- Open More options for climate, insulation and kitchen
- Read the cooling power and the standard size to buy
How it is worked out
- Base: about 20 BTU/h for each square foot (about 215 BTU/h per m²)
- Ceiling: scaled from an 8 ft (2.44 m) ceiling
- Sun: −10 % in a shaded room, +10 % in a very sunny room
- Climate: −5 % in cool summers, +10 % in hot, +20 % in very hot
- Insulation: −5 % for good, +10 % for poor
- People: +600 BTU/h for each person above two
- Kitchen: +4,000 BTU/h for the heat of cooking
The result is rounded up to the next 500 BTU/h. 1 kW ≈ 3,412 BTU/h, and 1 ton = 12,000 BTU/h.
Room size and cooling power
For a normal room with a 2.5 m ceiling, average sun, two people and average insulation:
| Room area | In sq ft | Cooling needed | In kW | Standard unit |
|---|---|---|---|---|
| 10 m² | 108 | 2,500 BTU/h | 0.7 kW | 5,000 BTU |
| 15 m² | 161 | 3,500 BTU/h | 1.0 kW | 5,000 BTU |
| 20 m² | 215 | 4,500 BTU/h | 1.3 kW | 5,000 BTU |
| 25 m² | 269 | 6,000 BTU/h | 1.8 kW | 6,000 BTU |
| 30 m² | 323 | 7,000 BTU/h | 2.1 kW | 7,000 BTU |
| 40 m² | 431 | 9,000 BTU/h | 2.6 kW | 9,000 BTU |
| 50 m² | 538 | 11,500 BTU/h | 3.4 kW | 12,000 BTU |
A 300 sq ft room (the English starting value) needs about 6,000 BTU/h.
Standard sizes
Window and wall units are sold in steps such as 5,000, 6,000, 8,000, 10,000, 12,000, 14,000, 18,000, 24,000 BTU/h. Split units are often sold as 2.0, 2.5, 3.5, 5.0, 7.0 kW. The calculator picks the smallest size that is not below your need.
Tips for a good choice
- If your number is between two sizes, take the bigger one only if the room is very sunny or hot; otherwise take the smaller
- An inverter unit adjusts its power and handles a slightly big unit better
- For an open plan or two rooms joined together, add their areas
- Close the curtains, seal gaps and add shade to reduce the need
- Check the energy label: a higher efficiency class saves money every year
Frequently asked questions
What size air conditioner do I need?
For an average room, about 20 BTU/h per sq ft (215 BTU/h per m²). The calculator adjusts this for your ceiling, sun, people and more.
How many BTU for 300 square feet?
About 6,000 BTU/h. For 500 sq ft it is about 10,000 BTU/h.
What does a ton of AC mean?
One ton of cooling is 12,000 BTU/h (about 3.5 kW) — the heat that melts a ton of ice in a day.
Can I use it for heating?
Heat pumps work for both. A rough guide: heating needs a similar size in mild climates, but cold climates need more. Ask an installer.
Is a bigger air conditioner better?
No. An oversized unit switches on and off all the time, does not remove the moisture, and costs more to run.
Is this an installer's calculation?
No. It is a good first guide. A full "load calculation" counts walls, windows and the building in detail.
Good to know
- Works in square metres or square feet — choose at the top
- Your numbers are saved only in this browser
- Nothing you type is sent anywhere
Related tools
- Electricity Cost Calculator — what it costs to run
- Solar Panel Calculator — power it from the roof
- Square Footage Calculator — measure your room
- Unit Converter — BTU, kW and more
- Paint Calculator — plan the other room jobs
