How to use this AC size calculator
- Room area, enter the floor area and pick square feet or square metres.
- People, enter how many people normally use the room.
- Sun exposure, choose Normal, Very sunny or Shaded for the room.
- Read the required cooling, tonnage and suggested AC size, they update instantly as you type.
Nothing is submitted or stored: the numbers never leave your device, so you can size units for several rooms privately.
How to size an air conditioner
Sizing comes down to matching the cooling output of the unit to the heat load of the room. The standard starting point is a rule of thumb of about 20 BTU per square foot of floor area. A 250 square foot bedroom therefore needs roughly 5,000 BTU per hour, while an open 600 square foot living space needs closer to 12,000 BTU. If you measured in square metres, this tool converts to square feet first by multiplying by 10.764.
Two adjustments matter most. People give off body heat, so the calculator adds about600 BTU for each person over two who regularly uses the space. Sun and extra heat sources also count: add roughly 10% for very sunny rooms or kitchens, and take off about 10% for well shaded rooms. Once the BTU figure is set, converting to tonnage is simple because 1 ton equals 12,000 BTU per hour, so the tons value is just the BTU divided by 12,000.
Air conditioners are sold in set sizes, usually in half ton steps, so the suggested AC size rounds the raw tonnageup to the nearest 0.5 ton. A room that calculates to 1.2 tons points you to a 1.5 ton unit. Sizing up a little is normal, but going far oversized wastes energy and leaves the room humid, so aim for the nearest fit.
An estimate, not a final spec
These figures are a fast, sensible estimate for a typical room with average insulation and ceiling height. They do not replace a proper Manual J load calculation, which an HVAC professional uses to account for insulation quality, window area and direction, ceiling height, local climate and air sealing. Use this calculator to narrow the range and sanity check a quote, then confirm whole home sizing with a qualified installer.
Frequently asked questions
What size AC do I need for a 12x12 room?
A 12 by 12 foot room is 144 square feet, so at 20 BTU per square foot it needs about 2,900 BTU, which is well within a small 0.5 ton unit unless it is very sunny or busy.
How many BTU is one ton?
One ton of cooling equals 12,000 BTU per hour, so divide any BTU figure by 12,000 to get tons.
Why add BTU for extra people?
Each person radiates body heat, so the calculator adds about 600 BTU per person above two to keep the room comfortable.
Is my data uploaded?
No, everything is calculated on your device and nothing is sent anywhere.