AC Size Calculator — BTU by Room Size

Air conditioner capacity is measured as a rate of heat removal, in BTU per hour, and it must match the rate at which heat enters the space. The 20 BTU per square foot starting point is a compressed approximation of a real cooling load, which is the sum of conduction through walls, ceiling, and windows, solar gain through glazing, infiltration of outside air, and internal gains from occupants and equipment. Each adult adds roughly 600 BTU per hour, and a kitchen typically needs about 4,000 BTU extra for cooking appliances. The multipliers applied here stand in for those terms: ceiling height scales the conditioned volume, sun exposure adjusts solar gain by about plus or minus 10 to 15 percent, insulation quality adjusts conduction by roughly 10 to 20 percent, and the climate factor accounts for higher design temperatures and, in humid regions, latent load. Tonnage is a legacy unit where one ton equals 12,000 BTU per hour, derived from the 288,000 BTU required to melt one short ton of ice over 24 hours. Oversizing is the more common and more damaging error. An oversized unit satisfies the thermostat quickly and shuts off before it has run long enough to condense moisture out of the air, so the room feels cool and clammy and indoor relative humidity can settle above 60 percent, while frequent short cycling wastes energy and accelerates compressor wear. Undersized equipment runs continuously and fails on design days. Rule-of-thumb sizing routinely overshoots a properly calculated load by 20 to 50 percent because it cannot see window orientation, glazing type, duct location, or air tightness. Treat this output as a shortlist rather than a specification, and for a permanent installation use an ACCA Manual J load calculation. Running cost here assumes an EER near 10, 8 hours of operation per day, and 13 cents per kilowatt-hour; current US minimum efficiency for split systems is 13.4 or 14.3 SEER2 depending on region, so a modern unit will cost less to run than this estimate.

Mechanical air conditioning began as a manufacturing process control rather than a comfort technology. In 1902 Willis Carrier designed a system for the Sackett-Wilhelms lithographic plant in Brooklyn to hold humidity steady so that colour printing would register correctly, cooling the air only as a means to that end. Carrier went on to publish his psychrometric formulae in 1911, which gave engineers the mathematics to treat temperature and moisture together and made deliberate load calculation possible. The ton unit was inherited directly from the ice trade the new machines displaced, since customers already understood cooling capacity in terms of how much ice a plant replaced per day. Room air conditioners became a mass consumer product in the United States after the Second World War, and with rapid postwar housing construction came a demand for quick sizing methods, which is where the per square foot rules of thumb took hold. The Air Conditioning Contractors of America published Manual J as the residential load calculation procedure, now in its eighth edition released in 2011, and it is referenced by model energy codes and by Department of Energy and ENERGY STAR guidance as the correct method. Efficiency labelling developed alongside sizing: EER measures performance at a single rating point, SEER was introduced to represent a seasonal average, and the SEER2 metric with more realistic external static pressure assumptions took effect in the United States in 2023. Square-foot shortcuts survive because Manual J needs envelope details a homeowner rarely has to hand.