Size heating and cooling in BTUs for a room's dimensions, climate and insulation.
Heating and cooling loads are calculated from the room's floor area using the industry rule-of-thumb of 20 BTU/sq ft for cooling and 25 BTU/sq ft for heating. Three multipliers adjust for climate severity, insulation quality, and sun exposure. Rooms taller than 8 feet have a proportionally higher load. Each occupant adds approximately 600 BTU/h to the cooling load. The result is converted to AC tonnage (12,000 BTU/h = 1 ton) and the next available standard unit size is recommended.
Cooling BTU
Cooling BTU = area × 20 × climate × insulation × sun × (height/8) + occupants × 600
Tonnage
Tonnage = Cooling BTU ÷ 12,000
BTU stands for British Thermal Unit — the amount of heat needed to raise one pound of water by 1°F. HVAC systems are rated in BTU/h (how many BTUs they move per hour).
1 ton of air conditioning = 12,000 BTU/h. The term originates from the cooling power of melting one ton of ice in 24 hours.
The rule-of-thumb method is sufficient for single-room sizing. For a whole house, a licensed HVAC contractor should perform a Manual J calculation that accounts for local design temperatures, window areas, duct losses and other factors.
Always round up for cooling — an undersized unit runs constantly and never achieves setpoint. For heating, rounding up is also safer. The calculator suggests the next standard size above the calculated load.