How the cooling estimate works
Air conditioners are rated by how much heat they can remove per hour, measured in BTU (British Thermal Units). The starting point for sizing a room unit is a rule of thumb: about 20 BTU per hour for every square foot of floor space. The calculator multiplies your room area by this base figure and then nudges it up or down for two factors that change the heat load.
A hot southern climate adds to the load while a cool northern one reduces it, and a room flooded with afternoon sun needs more capacity than a heavily shaded one. The result is a target cooling capacity in BTU per hour, which is also shown in "tons" — 12,000 BTU per hour equals one ton of cooling.
Why bigger is not better
It is tempting to round up to the largest unit you can afford, but an oversized air conditioner actually performs worse. It cools the air to the thermostat setting so quickly that it shuts off before it has run long enough to pull moisture out of the air.
The result is a room that feels cold and clammy rather than cool and dry, plus more wear from frequent on-off cycling and higher energy bills. Matching the capacity to the actual load keeps the unit running in longer, efficient cycles that manage both temperature and humidity.
Factors this rule of thumb ignores
The 20-BTU rule is a quick first pass, not a substitute for a proper load calculation. Several things can move the real number significantly:
- Ceiling height — the rule assumes a standard 8-foot ceiling; taller rooms hold more air to cool.
- Insulation and air sealing — a leaky, poorly insulated room needs more capacity.
- Windows — their number, size, orientation, and glazing strongly affect heat gain.
- Occupancy and appliances — people, kitchens, and electronics all add heat.
When to get a professional sizing
Use this estimate to choose a window or portable unit or to sanity-check a quote. For a whole-house or central system, have an HVAC professional run a Manual J load calculation, which accounts for the full building envelope, ductwork, and local design temperatures. The numbers here are a guide, not an engineering specification.
Formula
BTU/hr = area(ft²) × (20 + climate adjust + sun adjust); tons = BTU/hr / 12000Frequently asked questions
- Is bigger always better?
- No. An oversized unit cools quickly but cycles off before removing humidity, leaving the room clammy. Match the capacity to the load.
- What is a ton of cooling?
- One ton equals 12,000 BTU per hour, originally the heat needed to melt a ton of ice in a day.