About the BTU Calculator
A British thermal unit, or BTU, is a measure of heat, and air conditioners are rated in BTU per hour. Buy one that is too small and the room never cools; buy one that is too large and it switches on and off, cools unevenly and wastes electricity. This BTU calculator starts from a widely used room-size chart and adjusts for a sunny or shaded room, extra people, a kitchen and tall ceilings. It shows the answer in BTU per hour, kilowatts and tons of refrigeration, with every adjustment listed so you can see how it was reached.
Updated October 2026
How to use the BTU Calculator
- 1Choose square feet or square metres.
- 2Enter the room area and the ceiling height.
- 3Pick the amount of sunshine the room gets.
- 4Enter how many people normally use the room, and say if it is a kitchen.
- 5Read the BTU per hour needed and compare it with air conditioner labels.
How the estimate is built
The base figure comes from a published chart of capacity against room area: about 5,000 BTU for rooms up to 150 square feet, rising to 9,000 for 400 square feet, 12,000 for 550 square feet and 18,000 for 1,000 square feet. A very sunny room adds 10 percent and a heavily shaded one subtracts 10 percent. Each person beyond two adds 600 BTU. A kitchen adds 4,000. Ceilings above 8 feet scale the total in proportion to their height.
Ways to reduce the size you need
- Fit blinds, curtains or reflective film on sun-facing windows.
- Seal gaps around windows and doors.
- Improve attic and wall insulation.
- Use LED lighting, which gives off far less heat than old bulbs.
- Run heat-producing appliances in the cooler part of the day.
Choosing and running a unit
Pick the closest standard size at or just above the result. Check the efficiency label, because two units with the same BTU can differ greatly in running cost. Use the Electricity Bill Calculator and the kWh Calculator to estimate costs, and the Area Calculator to measure odd-shaped rooms. This tool gives a general estimate, and an installer can advise on your building and climate.
Features
- Base size from a room-area chart
- Adjustments for sun, people, kitchen and ceiling height
- Result in BTU/hr, kilowatts and tons
- Step-by-step list of the adjustments
- Square feet or square metres
- Rooms from small bedrooms up to 2,500 sq ft
- Runs in your browser
Frequently asked questions
How many BTU do I need for my room?
A common guide is about 20 BTU per square foot, though real charts give a little more for small rooms and a little less for large ones. A 400 square foot room with an 8 foot ceiling needs about 9,000 BTU per hour. Add more for strong sun, kitchens, extra people or tall ceilings.
What happens if the air conditioner is too big?
An oversized unit cools the air so quickly that it switches off before it has removed the humidity. The room feels cold and clammy, the unit cycles on and off often, and it uses more energy and wears out sooner. It is better to match the size closely.
How do I convert BTU to kilowatts and tons?
Divide BTU per hour by 3,412 to get kilowatts of cooling, and by 12,000 to get tons of refrigeration. A 12,000 BTU unit is one ton, which is about 3.5 kW of cooling. The result panel shows both for the size you need.
Does a kitchen need extra cooling?
Yes. Ovens, hobs and other appliances give off a lot of heat, so common sizing guidance adds about 4,000 BTU per hour for a kitchen. The tool includes that adjustment when you select Yes.
How is BTU different from the power a unit uses?
BTU is the cooling output, not the electricity input. A modern air conditioner with a seasonal efficiency of 3 or more produces three or more units of cooling for each unit of electricity. For running costs, check the efficiency rating and use the Electricity Bill Calculator.
Is this accurate for a whole house?
No. The method is for single rooms up to 2,500 square feet. Whole-house systems need a proper load calculation that accounts for insulation, windows, orientation, climate, ductwork and more. Ask an installer for one.