Sizing an air conditioner
Air conditioners are rated in BTU per hour — the amount of heat they can move out of a room. A widely used rule of thumb is about 20 BTU per square foot of floor area for cooling. This tool converts your room to square feet, multiplies by that figure, and adjusts for how sunny the room is and how many people use it, since both add heat.
Why getting the size right matters
An undersized unit runs constantly and never quite cools the room; an oversized one cools too fast, cycles on and off, and leaves the air damp and clammy because it doesn't run long enough to remove humidity. Matching capacity to the room gives steady, efficient comfort.
Adjust for your conditions
The 20-per-square-foot figure assumes average ceilings and insulation. Rooms with tall ceilings, lots of glass, a kitchen, or heavy sun need more; well-shaded, well-insulated rooms need less. The editable BTU rate and the sun and occupant options let you tune the estimate to your space.
Frequently asked questions
How many BTU do I need per square metre?
Roughly 215 BTU per square metre at the common 20-per-square-foot rate. The tool converts automatically when you choose square metres.
What is a BTU in kilowatts?
About 3,412 BTU per hour equals 1 kilowatt. The note shows the approximate kW so you can match it to units sold in kW.
What is a BTU?
A British Thermal Unit — the energy to raise one pound of water by 1 °F. It measures heating and cooling capacity for air conditioners and heaters.
How many BTUs do I need to cool a room?
A common guide is about 20 BTU per square foot, adjusted up for sun, high ceilings or many people. The tool sizes it from your room's area.
What happens if the BTU rating is wrong?
Too few BTUs and the unit never keeps up; too many and it cools in short bursts without removing humidity, leaving the room clammy. Correct sizing matters.