What this calculator does
This tool provides a rough estimate of the boiler output (heating capacity) needed to heat a home, based on floor area, climate zone, and insulation quality.
The formula
Total heat load = area × a BTU-per-square-foot factor that scales with climate severity (roughly 30 BTU/sq ft in mild climates up to 60 BTU/sq ft in cold climates), adjusted by an insulation quality multiplier. The result is also shown in kilowatts (1 kW ≈ 3,412 BTU/hr).
This is a simplified rule-of-thumb sizing guide for general planning. Actual boiler sizing should be based on a proper heat loss calculation performed by a licensed HVAC professional, accounting for windows, ceiling height, and local building codes.
Last reviewed August 2026