BTU SizerCooling size calculators
Same room, six different machines, six different answers. Each one carries the rule it is actually sized by.
Air conditioner size calculator
Room in, BTU out — and the nearest size that is actually sold.
Evaporative cooler size calculator
CFM from room volume — and the window that has to stay open.
Server room cooling calculator
Every watt in becomes a watt of heat — the rest is arithmetic.
Central AC tonnage calculator
Whole-house tons — the number on the quote, checked before you sign it.
Mini split size calculator
Head capacity per room — and why you do not add them up for the outdoor unit.
Portable AC size calculator
The one type where the number on the box is not the number in the room.
Window AC size calculator
The unit sits in the wall it cools, so the sizing is the honest one.
Cooling is the half of this site where the rule of thumb is genuinely good enough. A window unit or a single mini-split head costs a few hundred to a few thousand, serves one room, and a rate per square foot with four corrections lands within a size of what a full load calculation would say. Where it stops being good enough is a whole-house system: there the ducts, the orientation and the window area matter more than the floor plan, and that is what an ACCA Manual J calculation is for.
These cover the machines people actually choose between. The differences between them are not marketing — a portable really does deliver less cooling into the room than its label implies, a central system is sized on a different basis, and an evaporative cooler is not sized on heat at all but on how fast it can replace the air in the room.
Frequently asked questions
Why does the same room need a different size in each calculator?
Because the machines lose capacity in different places. A window unit sits in the wall it is cooling and loses almost nothing. A portable stands inside the room, exhausts through a hose that radiates heat back in, and effectively delivers less than its label — so the same room asks for more. A central system serves several rooms through ducts that leak and gain heat, so it is sized on total conditioned floor area rather than one room.
Should I size for the whole house or room by room?
Room by room for anything that cools one room — window units, portables, individual mini-split heads. Whole-house floor area for a central system or a multi-zone outdoor unit. Adding room-by-room figures for a central system overestimates badly, because no two rooms peak at the same hour of the day.
What if my room is open to the next one?
Measure both. An archway or a missing wall means one thermal space, and sizing only the room the unit stands in guarantees it will run continuously and never satisfy the thermostat. If the opening is a normal doorway that stays shut, treat them separately.
Do these figures work outside the United States?
Yes. Switch the tool to m² and the answer comes back in kilowatts as well as BTU. The rate per square foot is a heat-transfer figure rather than a national convention — what changes between countries is which unit the catalogue prints, and the tool prints all three.
How we calculate this
Sources: