calcs.homes
calcs.homes
Enter the space size, ceiling height, insulation level, and the temperature rise you want, and this heating BTU calculator sizes a garage, shop, or space heater in BTU per hour and electric watts.
Estimate only — big doors, slab-on-grade floors, and wind exposure raise real heat loss.
Heat loss from a simple box scales with its air volume and how far you want to lift the temperature. We multiply floor area by ceiling height for the volume, multiply by the temperature rise, and multiply by an insulation factor — about 0.13 for a tight, insulated space up to 0.27 for bare block or metal walls. Dividing BTU by 3,412 gives the equivalent electric heater size in watts.
This treats the space as uniformly enclosed. A garage with a big uninsulated overhead door, a bare concrete slab losing heat to the ground, or a windward wall will run colder than the number suggests, so round up to the next heater size and favor the poor-insulation factor if you are not sure. For an attached, insulated bonus room, the tight factor is closer.
A typical 400–600 sq ft garage with average insulation and a 40°F rise needs roughly 45,000–65,000 BTU, or about a 10–18 kW electric unit.
Anywhere from about 45 BTU/sq ft for a tight, insulated space up to 100+ BTU/sq ft for an uninsulated garage in a cold climate — volume and temperature rise drive it more than floor area alone.
Electric is simplest to install for small, well-insulated spaces. Propane or natural gas makes more sense for large or leaky shops where the electric load would be impractical.