Estimate the air conditioning or heating capacity (in BTUs) needed to comfortably condition a room.
This calculator estimates the air conditioning (or heating) capacity, measured in BTUs per hour, needed to comfortably cool or heat a room based on its size, ceiling height, sun exposure, and number of occupants.
Base BTU = Room Area (sq ft) × 25 BTU, scaled for ceiling height above the standard 8 ft, then adjusted for sun exposure (±10–15%) and an extra 600 BTU for each occupant beyond two.
A 12 ft × 15 ft room (180 sq ft) with an 8 ft ceiling, moderate sun, and 2 occupants needs about 4,500 BTU/hr (180 × 25), roughly equivalent to a small window AC unit.
Undersized units run constantly without properly dehumidifying the room, while oversized units cycle on and off too quickly. When between two standard sizes, round up.
It typically starts from a base BTU-per-square-foot estimate for your room/home size, then adjusts for factors like ceiling height, insulation, climate, and sun exposure.
British Thermal Unit — a measure of heat energy. In HVAC sizing, it indicates how much heating or cooling capacity a system can deliver.
An oversized unit cools or heats too quickly and shuts off before properly dehumidifying or evenly conditioning the space, leading to inefficient short cycling and uneven comfort.