Sizing home comfort equipment comes down to BTU per square foot, but the multiplier is different for cooling and heating, and bigger is not better. An oversized unit short-cycles: it blasts to temperature, shuts off before it pulls the humidity out, and leaves you cold and clammy. Here is how to size each piece.
Cooling: about 20 BTU per square foot
A room air conditioner needs roughly 20 BTU per hour per square foot, so a 300 sq ft room lands near 6,000 BTU. Energy Star then nudges it: add about 10 percent for a sunny room, subtract 10 percent for a heavily shaded one, add 4,000 BTU for a kitchen, and 600 BTU for each regular occupant beyond two.
| Room | Square feet | Cooling (BTU/hr) |
|---|---|---|
| 10 x 12 ft bedroom | 120 | 5,000 |
| 12 x 15 ft bedroom | 180 | 6,000 |
| 15 x 20 ft living room | 300 | 8,000 |
| 20 x 20 ft great room | 400 | 10,000 |
The air conditioner BTU calculator applies the sun, kitchen, and occupancy adjustments and also gives the answer in tons (one ton = 12,000 BTU) for comparing central units.
Heating: 30 to 60 BTU per square foot by climate
Furnaces scale with how cold your winters get: about 30 to 35 BTU per square foot in the warm South, up to 50 to 60 in the coldest northern states, adjusted for how well the house holds heat. A well-insulated 1,500 sq ft home in a middle climate needs roughly 60,000 BTU of output.
Two catches when you shop: furnaces are advertised by input BTU, and the heat you actually get is input times efficiency (AFUE). A 60,000 BTU output need means an 80,000 BTU input furnace at 80 percent AFUE, or about 63,000 input at 96 percent. The furnace BTU calculator works in the region and insulation factors so you size to output, not the box's headline number.
Water heater: size by the busy hour, not the tank
Tank gallons are the everyday shorthand, but the number that prevents cold showers is the first-hour rating (FHR) on the yellow EnergyGuide label: how many gallons of hot water the unit delivers during a peak hour. Match FHR to your household's busiest hour (think back-to-back morning showers plus a dishwasher), not to the tank size. Electric models are sized about 10 gallons larger than gas for the same real output because they reheat more slowly. The water heater size calculator turns household size into both a tank size and the FHR to look for.
Damp basement? Size the dehumidifier too
Humidity control is its own sizing job. Dehumidifiers are rated in pints per day, and the rating standard changed in 2019: a unit labeled 35 pints today removes about what a 50 pint unit did under the old test, so ignore any chart that predates it. The dehumidifier size calculator uses the current DOE standard and factors in how damp the space actually is, not just its floor area.
The theme across all four: size to the real load and resist rounding way up. Right-sized equipment runs longer, quieter cycles, controls humidity better, and costs less to buy and run.