🔥 Furnace Size Calculator
Calculate the BTU output needed for your furnace based on square footage, climate zone, and insulation quality.
How to Calculate Furnace Size
Furnace size is measured in BTU (British Thermal Units) output. The required BTUs depend on your home’s square footage, climate zone, ceiling height, and insulation quality. Cold climates (Zone 5–7) need 50–60 BTU per sq ft, while moderate climates (Zone 3–4) need 30–40 BTU per sq ft.
Furnace Sizing Tips
Common residential furnace sizes are 40,000, 60,000, 80,000, 100,000, and 120,000 BTU. An oversized furnace short-cycles, wasting energy and causing temperature swings. For the most accurate sizing, get a Manual J load calculation from an HVAC contractor. Furnace efficiency (AFUE) also matters: a 96% AFUE furnace delivers more usable heat than an 80% model.