Gas Furnace Sizing Calculator
Input & output BTU for a furnace from the design heat loss and AFUE efficiency.
Furnaces are sold by INPUT BTU. A 100k input @ 95 AFUE delivers 95k. Chronic oversizing = short cycles, cold drafts, cracked heat exchangers.
Formula
Gas Furnace Sizing Calculator is a free furnace size for HVAC designers, technicians and energy auditors โ instant, accurate and 100% client-side, with the governing formula and reference shown next to the result so the number can be defended, not just quoted.
About Gas Furnace Sizing Calculator
Input & output BTU for a furnace from the design heat loss and AFUE efficiency. The calculation implements Output = loss ร (1+margin); Input = Output / AFUE (ACCA Manual J/S). Furnaces are sold by INPUT BTU. A 100k input @ 95 AFUE delivers 95k. Chronic oversizing = short cycles, cold drafts, cracked heat exchangers.
How to use Gas Furnace Sizing Calculator
- 1Enter Design heat loss in kW (From a heat-loss calc or ~40โ60 BTU/ftยฒ cold climate).
- 2Enter Furnace AFUE in %.
- 3Enter Sizing margin in % (ACCA allows up to 40% oversize max).
- 4Read Required output, Furnace input rating instantly โ no submit button needed.
- 5Need US units? Flip the SI/Imperial toggle and every field converts.
Why use Gas Furnace Sizing Calculator?
- โImplements the standard formula โ Output = loss ร (1+margin); Input = Output / AFUE
- โReference cited on-page: ACCA Manual J/S
- โOne-click SI โ Imperial toggle โ values convert in place, physics stays in SI
- โRuns entirely in your browser โ nothing uploaded, free forever
Frequently asked questions
What formula does the Gas Furnace Sizing Calculator use?+
It computes Output = loss ร (1+margin); Input = Output / AFUE, per ACCA Manual J/S. The formula is displayed under the result.
What should I keep in mind when using this calculator?+
Furnaces are sold by INPUT BTU. A 100k input @ 95 AFUE delivers 95k. Chronic oversizing = short cycles, cold drafts, cracked heat exchangers.
Is this based on ASHRAE methods?+
The calculations follow the standard ASHRAE/ISHRAE formulations cited under the result, simplified where industry practice does the same. For permit-grade design, confirm with the full code procedure.
Is the Gas Furnace Sizing Calculator free to use?+
Yes โ completely free, no sign-up, no limits. It runs client-side in your browser, so inputs stay private and results are instant even on slow connections.
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