BTULab

BTU Calculator

A free BTU calculator that estimates the cooling or heating capacity your room needs. For cooling it starts at 20 BTU per square foot, then scales the result for ceiling height, sun exposure, climate zone, occupant count, and kitchen heat load. For heating it uses a climate-driven BTU-per-square-foot factor — 25 in a hot zone, 35 in a moderate one, 50 in a cold one — because heating demand varies far more with outdoor temperature than cooling does. All arithmetic runs in your browser on the numbers you enter; nothing is uploaded or stored. Use the result as a first-pass sizing estimate — a convenient starting point, not a substitute for a licensed installer's Manual J load calculation, which accounts for insulation R-values, window quality, air infiltration, and ductwork losses that this simplified model does not.