Boiler BTU Sizing Calculator

Estimate the required heating output for a commercial or residential replacement boiler based on building area and construction quality.

Building square footage Total conditioned area

Ceiling height (ft) Average, if not 8-9 ft

Building insulation / construction Affects heat loss per sq ft

Climate zone Design outdoor temperature

System efficiency (AFUE / thermal eff.) Typically 85-95% for condensing

Safety margin Recommended 10-15%

Enter valid numbers for all fields.

Estimated boiler sizing

Estimated heat loss (output required)

With safety margin

recommended boiler input rating (BTU/hr)

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This calculator provides a simplified, square-footage-based heat loss estimate for preliminary sizing only. It does not replace a formal Manual J (residential) or ASHRAE (commercial) heat loss calculation, which accounts for window area, orientation, air infiltration, and other factors. Always verify sizing with a qualified engineer before final equipment selection, particularly for commercial or multi-zone applications.