Furnace BTU Calculator — Free Online Calculator
This furnace BTU calculator estimates the heating output your home needs based on square footage, climate zone, and insulation — then converts it into the furnace input rating to shop for using the unit's AFUE efficiency. Use it to size a new furnace, check a replacement quote, or see how a high-efficiency model changes the size you need.
Sizing air conditioning instead? This page sizes heating. For cooling capacity, use the HVAC BTU Calculator — or convert an existing rating with the Tonnage Calculator.
Enter your home details
Higher AFUE = more of the fuel becomes heat, so you can buy a smaller input rating for the same output.
Heating output your home needs
Required furnace input
71,000 BTU
Per square foot
45 BTU
Suggested furnace size
80,000 BTU input furnace
See the breakdown
This is a planning estimate. For permits, equipment warranties, and final installation, confirm with a full ACCA Manual J load calculation.
Furnace size by square footage (quick reference)
Moderate climate (45 BTU/sq ft), average insulation, 95% AFUE furnace. Cold climates typically move you up one size.
| Home size (sq ft) | Heating output needed | Furnace size (input) |
|---|---|---|
| Up to 800 | 36,000 BTU | 40,000 BTU |
| 800 – 1,200 | 54,000 BTU | 60,000 BTU |
| 1,200 – 1,600 | 72,000 BTU | 80,000 BTU |
| 1,600 – 2,000 | 90,000 BTU | 100,000 BTU |
| 2,000 – 2,500 | 112,500 BTU | 120,000 BTU |
| 2,500 – 3,000 | 135,000 BTU | 140,000 BTU or two-stage |
Figures shown are planning estimates. Confirm final sizing with a full ACCA Manual J load calculation before purchasing or installing equipment.
The formula, explained in plain English
Furnace sizing is two short steps: figure out how much heat the house loses on a cold day (the heating load), then figure out which furnace can deliver that heat after its efficiency losses. Here is exactly what this calculator does.
Why 30–60 BTU per square foot?
Heating need scales with how cold your winters get. A Georgia home may need 35 BTU/sq ft on a design day; a Minnesota home can need 55 or more.
Why divide by AFUE?
Furnaces are sold by input (fuel burned), but your house needs output (heat delivered). Dividing the load by AFUE bridges the two.
Input vs. output in practice
An 80,000 BTU input furnace at 80% AFUE delivers 64,000 BTU; the same input at 95% AFUE delivers 76,000 BTU — nearly a size class more heat.
Why round up, not down?
Furnaces come in fixed steps (usually 20,000 BTU apart). Rounding up covers design-day cold snaps; rounding down risks a furnace that never keeps up.
Worked examples
Three real-world scenarios — including how a high-efficiency furnace changes the size you buy.
Typical home — 1,500 sq ft, moderate climate
Moderate zone (45 BTU/sq ft) · average insulation · 95% AFUE furnace.
67,500 ÷ 0.95 = ≈ 71,000 BTU input
round up → 80,000 BTU furnace
Result: an 80,000 BTU input, 95% AFUE furnace — the most common size class installed in mid-size American homes.
Small, well-built home — 1,000 sq ft, warm climate
Warm zone (40 BTU/sq ft) · excellent insulation (×0.9) · 95% AFUE furnace.
36,000 ÷ 0.95 = ≈ 38,000 BTU input
round up → 40,000 BTU furnace
Result: the smallest standard furnace (40,000 BTU) is plenty — a common surprise for owners of tight, newer construction.
Cold-climate home — 2,200 sq ft: 80% vs. 95% AFUE
Cold zone (50 BTU/sq ft) · average insulation · comparing furnace efficiencies.
at 80% AFUE: 110,000 ÷ 0.80 = 137,500 → 140,000 BTU furnace
at 95% AFUE: 110,000 ÷ 0.95 = 116,000 → 120,000 BTU furnace
Result: the high-efficiency model needs a full size class less input for the same heat — smaller equipment, lower fuel bills, same comfort.
Frequently asked questions
Common questions about furnace sizing, BTUs, and AFUE.
How many BTUs of heat do I need per square foot?
Heating needs run from about 30 to 60 BTU per square foot depending on climate. Hot southern climates need roughly 30–35, moderate climates 40–45, and very cold northern climates 50–60. Insulation quality moves the number up or down about 10%.
What size furnace do I need for a 2,000 square foot house?
In a moderate climate at 45 BTU/sq ft, a 2,000 sq ft home needs about 90,000 BTU of heat output. With a 95% AFUE furnace that means roughly 95,000 BTU of input — so a 100,000 BTU furnace. In warm climates an 80,000 BTU unit often suffices.
What's the difference between furnace input and output BTU?
Input is the fuel energy a furnace burns; output is the heat it actually delivers. Output = input × AFUE. An 80,000 BTU input furnace at 95% AFUE delivers 76,000 BTU of heat. Furnaces are sold by input rating, which is why this calculator converts for you.
What does AFUE mean on a furnace?
AFUE stands for Annual Fuel Utilization Efficiency — the percentage of fuel converted into usable heat over a season. A standard furnace is about 80% AFUE; high-efficiency condensing furnaces reach 95–97%. Higher AFUE means you can buy a smaller input rating for the same delivered heat.
Is a bigger furnace always better?
No. An oversized furnace heats the air fast and shuts off — a pattern called short-cycling that causes uneven temperatures, wasted fuel, and extra wear on the heat exchanger and blower. A right-sized furnace runs longer, steadier, more comfortable cycles.
What furnace sizes are available?
Residential gas furnaces are typically sold in input ratings of 40,000, 60,000, 80,000, 100,000, and 120,000 BTU, with some brands offering 45,000, 70,000, or 140,000 BTU models. Calculate your required input, then round up to the nearest size your brand offers.
How accurate is this vs. a Manual J load calculation?
This tool gives a fast planning estimate from the factors that matter most: area, climate, insulation, and efficiency. A full ACCA Manual J calculation also accounts for windows, orientation, air infiltration, and duct losses — and is required for permits and equipment warranties. Use this to plan and a Manual J to install.
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