BTU Calculator

British Thermal Unit — the heat needed to raise a pound of water 1°F. For air conditioning, the rating is BTUs of heat removed per hour, so a 10,000 BTU unit pulls 10,000 BTU of heat out of the room hourly. Central systems use tons instead: 1 ton = 12,000 BTU/hr, so a '2-ton' system is 24,000 BTU — same measurement, different tradition.

Because cooling and dehumidifying happen together, and dehumidifying takes runtime. An oversized unit hits the target temperature in minutes and shuts off before pulling meaningful moisture from the air — the room ends up cold but damp, which feels worse than warmer-and-dry. The rapid on-off cycling ('short cycling') also wears the compressor and wastes energy. The right-sized unit runs long, steady cycles: cooler, drier, quieter, cheaper.

The big four, all included here: sun (a south/west-facing room with big windows needs ~10% more; a shaded room ~10% less), ceiling height (BTUs cool volume, so 10-foot ceilings need a quarter more than 8), people (each body beyond two adds ~600 BTU of heat), and kitchens (+4,000 flat for appliance heat). What no simple calculator sees: insulation quality and your climate — hot-humid regions should round up.

Room heaters use the same BTU logic and this gets you in range, though heating loads depend more on insulation and outdoor temperature. Whole-house HVAC is a different animal — proper sizing is a Manual J load calculation accounting for windows, ducts, orientation, and infiltration, and a contractor who quotes one (rather than 'rule of thumb per square foot') is the one taking your money seriously.