BTU Calculator

Cooling load (BTU/h) = Floor area (sq ft) × 20 × Insulation factor × Window factor
Heating load (BTU/h) = Floor area (sq ft) × ΔT (°F) × Insulation factor × Window factor
Tons of refrigeration = BTU/h ÷ 12,000

• 20 BTU/h per sq ftbaseline cooling load for a room with a standard 8 ft ceiling, calibrated to roughly a 20°F design ΔT
• ΔTdesign indoor-to-outdoor temperature difference, in °F, for heating
• Insulation factor1.3 poor, 1.0 average, 0.8 good
• Window factor1.2 heavily glazed, 1.0 average, 0.9 few windows
• 12,000 BTU/hone ton of refrigeration, the unit air conditioners are sold in
• 3,412 BTU/hone kilowatt, for cross-checking against metric equipment ratings

• Oversizing is the mistake that costs comfort. A unit too large reaches the thermostat setpoint quickly, shuts off, and never runs long enough to condense moisture out of the air, leaving the room cold and clammy while the compressor short-cycles itself toward an early failure. Where the answer sits between two available sizes, the smaller one usually performs better.
• Energy Star adds roughly 600 BTU/h for each occupant beyond two and about 4,000 BTU/h where the unit also serves a kitchen. Rooms taking strong afternoon sun want around 10% more, heavily shaded rooms about 10% less.
• Anything you are paying an installer for deserves an ACCA Manual J load calculation instead. That works from actual wall and window U-values, orientation, air infiltration and local design temperatures, and it regularly comes out 20% or more away from the area rule. Rules of thumb are for choosing a window unit.
• Heating ΔT is a design temperature difference, not today’s forecast: use the coldest indoor-to-outdoor swing your climate sees in a normal winter, commonly 40 to 70°F depending on region, not an average day. A furnace sized to an average day runs out of capacity on the coldest week of the year.