Too small won't cool. Too big short-cycles and feels clammy. Find the right BTU rating for your space.
The starting point is about 20 BTU per hour for every square foot of floor area. For ceilings taller than 8 feet the number is scaled up by height ÷ 8. A sunny room multiplies the result by 1.1 and a shaded room by 0.9. Each person beyond two adds about 600 BTU, and a kitchen adds about 4,000 BTU. The total is rounded to the nearest 100, and the recommendation is the smallest standard air conditioner size at or above it. Tons are BTU divided by 12,000.
No. An oversized unit cools the room quickly and then shuts off before it removes much humidity, leaving the room cool but clammy. Choose the closest standard size at or above your estimate.
This rule of thumb works for single rooms of roughly 150 to 2,500 square feet. Open floor plans, whole-house cooling, or poor insulation call for a professional load calculation.
No. It estimates cooling load only. Heating needs depend on climate, insulation, and the type of system.
Yes. Poor insulation, large west-facing windows, and very hot climates all raise your real needs. Adjust upward if your room is unusually hard to keep cool.