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Furnace Sizing Calculator

Work out furnace sizing instantly with clear inputs, formula shown and shareable results.

About Furnace Sizing Calculator

A furnace is catalogued by input rating, but the building needs output. Adding a modest sizing margin to the design heat loss gives required output; dividing by AFUE converts that to the input rating to specify. Natural gas at roughly 1030 BTU per cubic foot then gives the appliance gas demand, and a 50 °F temperature rise gives the blower airflow.

Formulas

Furnace input

Input = heat loss x (1 + margin) / AFUE

AFUE
Annual fuel utilisation efficiency as a fraction

Blower airflow

CFM = output BTU/h / (1.08 x temperature rise)

Worked example

With the default values below, Furnace Sizing Calculator returns Furnace input rating needed of 72,632. Change any input above and every figure here is recalculated in your browser.

Inputs used

Design heat lossminimum 1 BTU/h
60,000 BTU/h
Furnace AFUEaccepts 50–100 %
95 %
Sizing marginaccepts 0–60 %
15 %

Results

Furnace input rating needed
72,632 BTU/h
Required output
69,000 BTU/h
Natural gas demand
70.5 ft³/h
Supply airflow at 50 °F rise
1,278 CFM

Step by step

  1. 1Values used
  2. 2Furnace input
  3. 3Blower airflow
  4. 4Furnace input rating needed= 72,632 BTU/h
  5. 5Required output= 69,000 BTU/h
  6. 6Natural gas demand= 70.5 ft³/h
  7. 7Supply airflow at 50 °F rise= 1,278 CFM

Assumptions and limits

  • The result is derived only from the 3 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
  • Preliminary estimate only. Confirm appliance input, venting and gas supply capacity with a licensed installer against the applicable fuel gas code.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Preliminary estimate only. Confirm appliance input, venting and gas supply capacity with a licensed installer against the applicable fuel gas code.

Frequently Asked Questions

How is the Furnace Sizing figure worked out?

Input = heat loss x (1 + margin) / AFUE. A furnace is catalogued by input rating, but the building needs output.

How big should the sizing margin be?

Most guidance caps oversizing at 15-25 percent of design heat loss. Beyond that the furnace short-cycles and comfort and efficiency both fall.

What values do I need to enter?

This calculator takes 3 inputs: Design heat loss, Furnace AFUE, Sizing margin. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.

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