Heating BTU calculator by climate zone
Heating equipment is sized for the coldest typical day, so climate matters more than anything except insulation. The calculator starts from the rule-of-thumb ranges furnace maker Bryant publishes and maps them to the IECC climate zones:
| Climate zone | Typical places | BTU/h per sq ft | 1,500 sq ft house |
|---|---|---|---|
| 1–2 (warm) | Florida, Gulf Coast, south Texas, Arizona deserts | 30–35 | 45,000–52,500 |
| 3–4 (moderate) | Southeast, Mid-Atlantic, lower Midwest, Pacific NW | 40–50 | 60,000–75,000 |
| 5–6 (cold) | Northern Midwest, New England, mountains | 50–60 | 75,000–90,000 |
| 7–8 (very cold) | Northern Minnesota, Alaska | 55–60 | 82,500–90,000 |
Pick Newer, tight for the low end of the range, Older, drafty for the high end. The mapping of zones to Bryant's warm, moderate and cold bands is ours; the climate zone of every U.S. county is on the DOE's climate zone map.
What size furnace do I need?
The calculator's result is the heat the house needs — the furnace's output. Furnaces are sold by input rating, and only the AFUE share of the input reaches the house:
Furnace input = heat needed ÷ AFUE
| Furnace input | Output at 80% AFUE | Warm climate | Moderate | Cold |
|---|---|---|---|---|
| 40,000 BTU | 32,000 | 914–1,067 sq ft | 640–800 sq ft | 533–640 sq ft |
| 60,000 BTU | 48,000 | 1,371–1,600 | 960–1,200 | 800–960 |
| 70,000 BTU | 56,000 | 1,600–1,867 | 1,120–1,400 | 933–1,120 |
| 80,000 BTU | 64,000 | 1,829–2,133 | 1,280–1,600 | 1,067–1,280 |
| 100,000 BTU | 80,000 | 2,286–2,667 | 1,600–2,000 | 1,333–1,600 |
| 120,000 BTU | 96,000 | 2,743–3,200 | 1,920–2,400 | 1,600–1,920 |
A 95% furnace with the same input delivers about 19% more heat, so you can often go one size down when you upgrade from an old 80% unit.
Why a Manual J beats any rule of thumb
The Department of Energy's Building America guidance is blunt: size heating and cooling equipment with ACCA's Manual J load calculation and Manual S equipment selection, not by floor area. Rules of thumb ignore what actually drives heat loss — window area, insulation levels, air leakage, ceiling height and your local design temperature — and they tend to produce oversized equipment that short-cycles and wastes fuel.
Choosing the system itself? See heat pump vs gas furnace for heating cost by state. Use this calculator to sanity-check a quote: if a contractor proposes a furnace far above the range here, ask to see the Manual J. Often the cheaper fix is the envelope — the blown-in insulation calculator and insulation calculator show what it takes to bring an attic or walls up to today's levels. Heating just one room? The space heater cost calculator prices running an electric heater; the cooling side is in the BTU calculator.
