By Zencalcy Editorial TeamLast reviewed About us
A furnace must replace the heat a house loses at the coldest hour of the year. Floor area times a climate-and-insulation factor gives the heat loss estimate, and the market's 5,000 BTU sizing steps mean the answer is the next size up.
Furnace BTU from floor area, climate and insulation.
heat loss ≈ area × climate factor furnace BTU = heat loss rounded to the next 5,000
The factors — 13 BTU per sq ft for mild climates up to 30 for cold and drafty — assume a 0°F outdoor design temperature and 70°F indoors. Gas furnaces are sold in 40,000 to 120,000 BTU steps; oversizing causes short cycling that skips the second stage of efficiency.
| Symbol | Meaning |
|---|---|
| A | Heated floor area. |
| f | Climate and insulation factor. |
| Q | Heating load in BTU/hr. |
1,800 × 22 = 39,600 BTU/hr, so a 40,000 BTU furnace covers the load with a hair to spare.
Continue with the Paint Calculator, Wallpaper Calculator, Tile Calculator, or the Flooring Calculator — or browse more home & garden projects and all construction calculators.
About 13 to 15 in mild climates, 18 to 22 in average northern zones, and 25 to 30 in cold regions or leaky older houses.
Results use standard construction estimating arithmetic: volumes from measured area and depth, unit weights from published material densities, and code-referenced spans or clearances where the page names a code. Quantities are rounded up to whole saleable units and include the stated waste allowance. These tools are planning aids, not structural design, and do not replace a licensed engineer, a local code check, or a contractor's site measurement.
Reference material used to build and check this calculator:
Last reviewed: September 2026. See how we build and check every tool on the about page.
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Content updated: September 2026