How do I calculate furnace size? Enter your numbers above and the result updates instantly. For example: Home area: 2,000 sq ft · Climate zone: Cold (zone 5) · Furnace efficiency (AFUE): 96% → Required heat output: 100,000 BTU/hr.
Is this Furnace Size Calculator free? Yes — completely free with no sign-up or paywall. It runs entirely in your browser and keeps working offline; the numbers you enter never leave your device.
How accurate is it? It uses the standard published formula for furnace size. Results are estimates for planning — not financial, tax, medical, engineering, or legal advice.