Enter your home size, climate zone, sun exposure, and the number of people in the home. You get a recommended cooling range in BTU and tons, with the math shown so you can sanity-check any contractor quote.
Cooling equipment is sized in BTU per hour, and 12,000 BTU equals one ton. The cooling load is the heat your AC or mini-split has to remove to hold a comfortable temperature. It depends on floor area, your climate, how much direct sun the home takes, and the people and appliances inside, since each adds heat. The fast method multiplies cooled square footage by a climate-zone factor, adjusts for sun, and adds a little for extra occupants, which is why the calculator returns a range rather than one exact number.
A common mistake is buying a bigger unit to be safe. Oversizing backfires. An oversized AC cools the air fast, satisfies the thermostat, and shuts off before it has run long enough to pull moisture out of the air. That short-cycling leaves the home cool but clammy, wears the compressor with frequent starts, and wastes energy. A right-sized unit runs longer, gentler cycles that dehumidify well and hold a steady temperature.
For a sizing-critical install, especially a variable-speed system or a multi-zone mini-split, ask your contractor for a room-by-room Manual-J load calculation. It accounts for window area and orientation, ceiling height, insulation, and measured air changes, which a square-foot rule of thumb cannot. Use this calculator to land in the right range and to check that a quote is not wildly oversized, then let a Manual-J confirm the final tonnage.