Furnace Size Calculator

Furnace heating capacity is the rated output delivered by the equipment; input describes its fuel energy rate. This calculator compares a known design heat loss, or a simplified envelope estimate, with an illustrative input-size list using an entered rated output/input ratio. Seasonal AFUE is a different metric and must not be used as an exact rated-capacity ratio.

How to use this tool

1. Choose a known design heat loss or an envelope estimate.
2. For an estimate, enter area, stories, ceiling height, preset insulation and window area.
3. Enter winter and indoor temperatures and the assumed space below the floor.
4. Enter the manufacturer’s rated output divided by rated input, as a percentage.
5. Compare the arithmetic results with the actual model’s design-condition output.

Formula

Required input = design heat loss ÷ rated output/input ratio; illustrative output = input × ratio

The same entered ratio is applied to every listed size solely for comparison. The envelope estimate adds A × ΔT/R, window U × A × ΔT and 0.018 × volume × natural ACH × ΔT. It assumes a square footprint, fixed illustrative assembly values and no duct loss. A floor above a 50°F space is a suspended-floor scenario, not a ground-slab calculation.

Example

The default 2,000 sq ft envelope scenario gives about 41,945 BTU/h heat loss at 5°F outside and 70°F inside. If rated output/input is 95%, required input is about 44,153 BTU/h. A listed 40,000 input produces 38,000 output under that assumption; the first listed size meeting the load is 60,000 input, giving 57,000 output. That arithmetic alone does not establish acceptable oversizing or compliance.