Sizing a Gas furnace? A Gas furnace typically runs 80% AFUE as standard, with high-efficiency condensing models reaching 90–96%. Higher AFUE means a smaller input rating for the same delivered heat and lower fuel bills. For a typical 2,000 sq ft home in a moderate climate needing about 86,000 BTU/hr of heat, an 80% AFUE Gas furnace must be rated roughly 120,000 BTU input. The calculator below is pre-set to a Gas furnace — enter your home’s details for an exact size.
🔥 What Size Furnace Do I Need? — BTU & AFUE Calculator
Find the right furnace size for your home in BTU. Enter your square footage, climate zone, insulation and fuel type — we calculate the heat output you need and the furnace input rating (adjusted for AFUE efficiency) so you buy the correct 40k, 60k, 80k or 100k BTU unit. For homes in the US, Canada, UK & Europe.
🌍 Prefer metric? Switch units here
📐1. Floor Area & Ceiling
Range: 100 – 10,000 sq ft (whole-home or a single addition)
Range: 7 – 20 ft. Taller ceilings need proportionally more heat.
🌍2. Climate Zone
Zones follow US DOE / IECC heating climate zones. Colder zone = more BTU per sq ft. Pick your country above to auto-set it.
🧱3. Insulation
🪟4. Windows / Glazing
💨5. Air Tightness / Drafts
Older, draughty homes lose heat through air leaks and need a bigger furnace. A modern, sealed home needs less.
🔥6. Fuel Type & Furnace Efficiency (AFUE)
Standard gas = 80% · High-efficiency condensing = 90–98% · Electric = 100%. AFUE is the % of fuel turned into usable heat.
Gas Furnace Size by Home Area (80% AFUE)
Typical Gas furnace sizes for a moderate (Zone 4) climate, 8-foot ceiling and average insulation. The heat output needed is the same for any fuel; the input rating shown reflects an 80% AFUE Gas furnace:
| Home Size | Heat Output Needed | Gas Furnace Size |
|---|---|---|
| 1,000 sq ft | 43,000 BTU/hr | 60,000 BTU |
| 1,500 sq ft | 64,500 BTU/hr | 80,000 BTU |
| 2,000 sq ft | 86,000 BTU/hr | 120,000 BTU |
| 2,500 sq ft | 107,500 BTU/hr | 140,000 BTU |
| 3,000 sq ft | 129,000 BTU/hr | 160,000 BTU |
Gas Furnace Efficiency & Sizing
Furnace sizing has two numbers: the output (delivered heat your home needs) and the input (fuel the furnace consumes — the nameplate rating you buy by). Output = input × AFUE. Size your home to the output it loses on a cold day, then divide by your Gas furnace’s AFUE to get the input rating, and round up to a standard size. A high-efficiency condensing Gas furnace delivers the same heat from a lower input, saving fuel over the life of the unit. Enter your exact area, climate and insulation in the calculator above.
Frequently Asked Questions
What size Gas furnace do I need? For a typical 2,000 sq ft home in a moderate climate you need about a 120,000 BTU input Gas furnace (80% AFUE), delivering roughly 86,000 BTU/hr of heat. Scale with your home size per the chart above.
Does fuel type change the furnace size? The heat output your home needs is the same for any fuel. What changes is the input rating: a lower-AFUE furnace needs a bigger input for the same heat. A Gas furnace at 80% AFUE turns 86,000 BTU output into about a 120,000 BTU input rating.
What AFUE should a Gas furnace be? Standard Gas furnaces are around 80% AFUE; high-efficiency condensing models reach 90–96% and cut fuel use significantly in cold climates.
Estimates vary by contractor. Compare free quotes from licensed local pros before you commit. Request a quote →
High-efficiency systems may qualify for utility rebates and federal or state incentives. Find current programs at ENERGY STAR & DSIRE.
Figures are reviewed by the Refcon HVAC engineering team using U.S. DOE, ENERGY STAR & ACCA Manual J data. Estimates only — confirm with a licensed pro.