BTU Heating Calculator

About this calculator

Sizing a heater — whether a central furnace, a mini-split, or a portable space heater — starts with estimating how many BTUs per hour (British Thermal Units, the standard heating-capacity unit in the US and often converted to kW elsewhere) a room needs to stay warm. The rule-of-thumb multiplier is roughly 30 to 60 BTU/hr per square foot depending on climate zone, with colder regions, poor insulation, and higher ceilings all pushing the requirement upward, since taller rooms have more air volume to heat.

This calculator takes the room's floor area, ceiling height, and climate, and returns the estimated heating capacity needed in both BTU/hr and kW. HVAC installers, homeowners shopping for a space heater or mini-split, and contractors quoting heating jobs use it as a starting sizing estimate before consulting a full Manual J load calculation for a whole-house system.