calcs.homes
calcs.homes
Enter your room's area, ceiling height, sun exposure, and how many people use it, and this BTU calculator returns the cooling capacity in BTU per hour and a recommended air conditioner size.
Estimate only — a Manual J load calculation is the standard for a permanent system.
The base figure is 20 BTU per hour for each square foot of floor — the long-standing rule of thumb for a room with 8-foot ceilings. We scale that up for taller ceilings (more air volume), adjust ±10% for a room that is heavily shaded or gets strong afternoon sun, add 600 BTU for every occupant beyond two, and add 4,000 BTU for a kitchen because the appliances throw off heat. The result is rounded to the nearest common unit size.
This is fine for sizing a window or portable unit or a single-room mini-split. A whole-house system should be sized with a Manual J load calculation, which accounts for insulation levels, window type and area, duct losses, and local design temperatures — an oversized AC short-cycles, cools unevenly, and never runs long enough to pull humidity out of the air.
About 20 BTU per hour per square foot for a room with standard ceilings. A 300 sq ft room needs roughly 6,000 BTU; a 500 sq ft room about 10,000.
Around 6,000–8,000 BTU, depending on sun, ceiling height, and how many people are in it. Bump up 4,000 BTU if it is a kitchen.
No. An oversized unit cools the air fast, shuts off, and leaves the room clammy because it never runs long enough to dehumidify. Right-sizing matters more than raw capacity.