BTU Calculator
Enter the room size to estimate the air conditioner size in BTU.
What this tool does
Air conditioners are rated in BTU per hour. A common room-sizing chart suggests about 7,000 BTU for a room of 250-300 square feet, 8,000 for 300-350 and 12,000 for 450-550. Increase it by 10% for a very sunny room, add 600 BTU for each person beyond two and 4,000 for a kitchen. Enter your room above.
Looks up a base capacity for the room size from a published rule-of-thumb chart.
Adjusts for sunlight, occupants and a kitchen, and converts the result to kW and tons.
How to use it
- Choose square feet or square metres.
- Enter the room area.
- Pick the sunlight level, the number of people and whether it is a kitchen.
- Read the BTU per hour needed.
Why use this calculator
Suggests an air conditioner size in BTU per hour from the room area, with adjustments for sun, extra people and a kitchen, and converts it to kilowatts and tons of cooling. An air conditioner that is too small never cools the room, and one that is too big switches on and off, wastes electricity and removes less moisture. The room size is the starting point for choosing.
Why we built it
We built the BTU calculator on a standard room-size chart with adjustments for sun, people and a kitchen. Estimate running costs with the electricity cost calculator and convert power with the power converter.
Who this tool is for
- Homeowners buying an air conditioner
- Renters sizing a window unit
- Installers doing a quick estimate
When to use it
- Buying a window or portable air conditioner.
- Comparing units in a shop.
- Replacing an old unit.
- Estimating running costs.
How it works
The base capacity comes from a standard chart of room area against cooling capacity: 100-150 sq ft needs about 5,000 BTU, 150-250 sq ft 6,000, 250-300 sq ft 7,000, 300-350 sq ft 8,000, up to 2,000-2,500 sq ft at 34,000. Then ±10% for sun, plus 600 per person beyond two and 4,000 for a kitchen. 1 BTU/h = 0.29307 W, and 12,000 BTU/h is one ton of cooling.
Example: a 300 sq ft room is 7,000 BTU, which is 2.05 kW; very sunny, it is 7,700 BTU; with four people, 8,200 BTU.
A rule of thumb for one room with an 8 ft ceiling. Ceiling height, insulation, windows, climate and open doorways change the need, so ask an installer for important decisions.
A worked example
These are the values the calculator opens with, so you can see exactly what it does with them. Change any value on the page and the result updates; the figures below come from the same calculation.
| Input | Value |
|---|---|
| Room size in | Square feet |
| Room area | 300 |
| Sunlight | Normal |
| People who usually use the room | 2 |
| Is it a kitchen? | No |
With those inputs, the calculator returns:
- Cooling needed: 7,000 BTU/hour
- Base size for the room: 7,000 BTU/hour
- Sunlight adjustment: +0%
- Extra for people and kitchen: +0 BTU/hour
- In kilowatts: 2.05 kW
- In tons of cooling: 0.58
What you give it, and what you get back
| Item | Details |
|---|---|
| Room area | Square feet or square metres, 100-2,500 sq ft. |
| Conditions | Sunlight, people and kitchen. |
| Output | BTU per hour, kW and tons, with the adjustments shown. |
Issues you may run into
| What you see | Why it happens | What to do |
|---|---|---|
| Room outside the chart | The chart covers 100 to 2,500 sq ft. | For larger spaces, ask an installer. |
| The unit seems too small | Open plan layouts need more. | Add the connected areas. |
| Cooling and heating | This is for cooling. | Heating needs differ. |
Assumptions and limitations
- A rule-of-thumb chart, not a load calculation.
- Standard ceiling height assumed.
Frequently asked questions
How many BTU do I need for a room?
About 7,000 BTU for a 250-300 sq ft room, rising with size.
How do I convert BTU to kW?
Multiply BTU per hour by 0.000293.
What is a ton of cooling?
12,000 BTU per hour.
What if the room is very sunny?
Add about 10%.
Is bigger better?
No. An oversized unit cools unevenly and wastes energy.
Is my information sent anywhere?
No. The calculation runs in your browser and nothing leaves the page.
Sources and accuracy
Cooling capacity is estimated from room area using the United States Department of Energy rule of thumb of about 20 BTU per square foot, adjusted for occupancy and conditions entered by the user. Efficiency ratings and sizing guidance are published by ENERGY STAR.
Figures last checked against the source on . Standards and rates can change — if you are relying on this for something that matters, confirm against the source above. Think a figure is wrong or out of date? Send a correction.