BTU Heating Calculator
Estimate the heating BTUs a room needs — by the quick climate rule of thumb (area, climate, ceiling height, insulation, sun) or a heat-loss estimate from the indoor and outdoor temperatures, walls, windows, ceiling, floor and air leakage — in BTU/h, kW, watts of electric heat and furnace input.
How it works
Quick: BTU/h = area × climate factor × (ceiling ÷ 8 ft) × insulation factor × sun factor · Heat loss: BTU/h = (Σ U × area of walls, windows, doors and ceiling + F × slab edge + ½ × U × floor + 0.018 × air changes × volume) × (indoor − outdoor °F) · kW = BTU/h ÷ 3,412 · tons = BTU/h ÷ 12,000 · furnace input = load ÷ AFUE
The quick estimate scales a room’s heating load with its floor area and the climate: from about 22 BTU per hour per square foot in hot southern regions, 30 in moderate ones and 40 in cold northern ones, up to 50–60 in very cold and subarctic areas. Taller-than-standard ceilings mean more air to heat, so the load is scaled by the ceiling height against an 8 ft baseline, then adjusted down about 15% for a tight, well-insulated room or up 25% for a drafty one, and ±10% for sun or shade. The heat-loss estimate works it out from physics instead: each surface loses its U-value × area × the temperature difference between your indoor setting and the outdoor design temperature (a typical coldest-night figure for your area), a slab loses heat along its exposed edge, and leaking air takes 0.018 BTU per cubic foot per °F each time the room’s air is replaced. The U-values are typical figures for each kind of wall, window and attic insulation, in line with the IECC code tables. Dividing BTU/h by 3,412 gives kilowatts — and watts of electric heat — and by 12,000 gives the “ton” used for heat pumps. A fuel furnace delivers only its AFUE share of its input rating, so the input to buy is the load divided by the efficiency.
Worked example
A 300 ft² room in a moderate zone (30 BTU/ft²) with standard 8 ft ceilings: height factor 8 ÷ 8 = 1, so 300 × 30 × 1 = 9,000 BTU/h — about 2.6 kW, two 1,500 W heaters, or 0.8 tons. The heat-loss method for the same room at 68 °F inside and 5 °F outside (ΔT 63 °F), with 35 ft of R-13 exterior wall, 30 sq ft of double-pane windows, an R-30 attic, an insulated crawlspace and average air leakage: walls 250 × 0.084 = 21.0, windows 30 × 0.5 = 15.0, ceiling 300 × 0.035 = 10.5, floor 300 × 0.05 × ½ = 7.5 and air 0.018 × 0.7 × 2,400 = 30.2 BTU/h·°F, a total of 84.2 × 63 = 5,307 BTU/h.
Sources: 2021 International Energy Conservation Code — Table R402.1.2 (U-factor equivalents) · US Department of Energy — Furnaces and boilers (AFUE)
Frequently asked questions
Is this the same as a Manual J load calculation?
No. The quick estimate is a rule of thumb for a single room, and the heat-loss estimate is a simplified version of the same physics. A full ACCA Manual J also accounts for window orientation, duct losses, internal gains and your exact local design temperatures. Use this to get in the right ballpark, then have an HVAC pro run Manual J before buying equipment for a house.
How many BTUs do I need per square foot?
About 20–25 BTU/h per square foot in hot climates, 30–35 in moderate ones, 40–45 in cold northern ones and 50–60 in very cold areas, for an average room with 8 ft ceilings. Well-insulated modern rooms often need less; the heat-loss method shows how much your room really loses.
What lowers a room’s heating load?
Better insulation, tight windows and doors, and sealed air leaks all cut the BTUs needed. In the heat-loss breakdown, air leakage and windows are often the biggest items — sealing gaps and upgrading single-pane glass can halve the load of an old room.
Why does ceiling height matter?
Heating warms the air volume, not just the floor. A room with 12 ft ceilings holds 50% more air than the same footprint at 8 ft, so it needs proportionally more heat — which is what the height factor captures, and why the heat-loss method counts wall height and air volume.
What size electric heater do I need?
Divide the BTU/h by 3.412 to get watts: 9,000 BTU/h is about 2,640 W, so two 1,500 W heaters. A plug-in heater tops out at 1,500 W (5,118 BTU/h) on a standard 120 V circuit, so larger rooms need several heaters or a hard-wired 240 V heater.
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Estimate for planning. Add a waste margin and confirm with your supplier and local codes.