How btu room sizing calculator works
The starting point is the classic rule of thumb: a moderately insulated room with standard 8-foot ceilings needs roughly 20 BTU per hour per square foot for cooling. That baseline is then adjusted for the things that change real heat gain — sun, insulation quality, ceiling height, people, and kitchen appliances.
Sunny rooms gain 10% and shaded ones lose 10%. Poor insulation multiplies the load by 1.2 because heat leaks both directions; good insulation trims it to 0.9. Taller ceilings scale the load by height ÷ 8, since air volume — not just floor area — has to be conditioned. Each occupant beyond the second adds about 600 BTU of body heat, and a kitchen adds 4,000 for cooking appliances.
Heating is reported separately because it depends on climate more than on sun: 30 BTU per square foot in warm regions, 40 in temperate, and 50 in cold, then the same insulation and height factors. Results are rule-of-thumb estimates for room-sized units — whole-house sizing uses a different, more rigorous load calculation.