Air Conditioner Size Calculator (BTU and Tons)
Find what size air conditioner a room needs. Enter the room size, insulation, sun, occupants and kitchen to get cooling BTU per hour, tons and kilowatts, plus an optional heating load.
The first two are free; each extra person adds 600 BTU/h.
How many degrees to warm the room. Blank hides the heating load.
Enter the room length, width and ceiling height.
About the air conditioner size calculator
The Air Conditioner Size Calculator answers the question "what size air conditioner do I need for this room?" Enter the length, width and ceiling height, say how well the room is insulated and how much sun it gets, add the number of people and whether it contains a kitchen, and you get a cooling capacity in BTU per hour. The same figure is shown in tons and kilowatts, which are the units you will see on window units, portable units and mini split spec sheets.
Every dimension has its own unit, so you can type a room 4 m wide and 3 m long with a ceiling given in feet, if that is how you measured it. A separate heating figure appears when you enter a temperature rise, which helps if you are shopping for a heat pump or a reversible mini split.
How to find the right air conditioner size
Steps
- Measure the room length, width and ceiling height. Use the room the unit will actually cool, not the whole home.
- Pick insulation: poor for an uninsulated loft or old single-glazed room, good for a well-sealed modern room.
- Pick sun exposure. A shaded room lowers the load, a sunny one raises it.
- Set the number of occupants and tick kitchen if cooking heat is present.
- Compare the BTU/h result with the capacity printed on the units you are considering.
How the size is calculated
Cooling load
is floor area in square feet. is height divided by 8 ft above 8 ft, otherwise 1. is 1.1, 1.0 or 0.9 for poor, average or good insulation, and is 0.9, 1.0 or 1.1 for shaded, average or sunny. is occupants, and is 1 when the room has a kitchen. One ton is 12,000 BTU/h, and kilowatts equal BTU/h x 0.00029307107.
Worked examples
Small shaded bedroom. A room 12 ft by 10 ft with an 8 ft ceiling, good insulation, shaded, and 1 occupant is 120 square feet. The load is 120 x 20 x 0.9 x 0.9 = 1,944 BTU/h, or 0.16 ton and 0.57 kW. That is below the smallest common window unit, so a 5,000 BTU/h model would already be larger than the estimate.
Sunny living room. A room 18 ft by 14 ft with an 8 ft ceiling, average insulation, sunny, and 3 occupants is 252 square feet. The base is 5,040 BTU/h, sun takes it to 5,544, and one extra person adds 600, for 6,144 BTU/h. That is 0.51 ton or 1.80 kW, so a 6,000 to 8,000 BTU/h window unit would be in range.
Metric room. A room 4 m by 3 m with a 2.7 m ceiling is about 129.17 square feet with an 8.86 ft ceiling. The height factor is 1.107, so the load is about 2,860 BTU/h, 0.24 ton or 0.84 kW, for average conditions and 2 occupants.
Choosing an actual unit
Window units usually run from about 5,000 to 12,000 BTU/h, and larger rooms often use a mini split or a central system rated in tons. One ton is 12,000 BTU/h. Pick a unit close to the estimate rather than far above it, because one that is 10 to 20 percent oversize can short-cycle, switching off before it dehumidifies properly. If the result falls between two sizes, consider the room's weakest point, such as a west-facing window, before going up. The wider tool behind this page is the BTU Calculator.
Accuracy and limits
This is an estimate from rules of thumb. Climate zone, window area, air leakage and duct losses are not modelled. A Manual J load calculation by a contractor is the correct basis for sizing equipment. For a quick conversion of your measurements, the Length Converter handles feet, metres and inches, and the Area Calculator finds floor area for unusual room shapes.
Privacy
Nothing is uploaded; every calculation runs in your browser and never leaves your device.
Frequently asked questions
What size air conditioner do I need for my room?
Start from 20 BTU/h per square foot of floor area, then adjust for ceiling height, insulation, sun and occupants. A 15 x 12 ft room with an 8 ft ceiling and average conditions comes to 3,600 BTU/h. The calculator applies all the adjustments and shows the answer in BTU/h, tons and kilowatts.
What is a ton in air conditioning?
One ton of cooling is 12,000 BTU/h, which is about 3.52 kW. Central systems and larger mini splits are usually described in tons, while window units are described in BTU/h.
What if my result is smaller than the smallest window unit?
Window units commonly start around 5,000 BTU/h. For a small, shaded, well-insulated room the estimate can be lower, for example 1,944 BTU/h for a 12 x 10 ft bedroom. In that case choose the smallest efficient unit you can find rather than a much larger one.
Should I round up to the next size?
Only if the estimate sits just under a standard size. A unit that is 10 to 20 percent oversize can short-cycle, shutting off before it removes much humidity. A much larger unit is a worse choice than one that is slightly under.
Why does a sunny room need more cooling?
Sunlight through windows and onto walls and the roof adds heat. The calculator multiplies the load by 1.1 for a sunny room and by 0.9 for a shaded room, so shade lowers the result and strong sun raises it.
Can I use this to size a whole house system?
No. It is an estimate for a single room. A whole-house system must account for climate zone, window area, air leakage and duct losses, which this tool does not model. Ask a contractor for a Manual J load calculation before buying equipment.
Is my data sent anywhere?
No. The sizing sums run in your browser, so your room dimensions and choices are never uploaded or stored on a server. They stay on this device and are not shared with anyone.