Last updated June 18, 2026 by our expert review team
What Size Furnace Do I Need?
Most homes need a 40,000–120,000 BTU furnace — about 30 BTU per square foot in hot climates and up to 60 in cold ones, then divided by the furnace's AFUE efficiency to get the input rating you actually buy. A 2,000 sq ft moderate-climate home lands near a 100,000 BTU furnace.
Recommended furnace size by home size and climate (95% AFUE, average insulation):
| Home size | Warm | Moderate | Cold |
|---|---|---|---|
| 1,000 sq ft | 40k | 60k | 60k |
| 1,500 sq ft | 60k | 80k | 80k |
| 2,000 sq ft | 80k | 100k | 120k |
| 2,500 sq ft | 100k | 120k | 140k |
| 3,000 sq ft | 120k | 140k | 140k |
Input BTU at 95% AFUE. An 80% furnace needs the next size up. Insulation, windows, and ceiling height shift these — the calculator uses your exact inputs.
Expert Contributors
Ehsan Ghazanfari
Licensed Structural Engineer
FISE-certified structural engineer with 11+ years designing bridges, retaining walls, and foundations. MSc from Aalto University.
See full profileHawkin
Certified Cost & Estimating Professional
AACE-certified estimator working with 20+ insulation companies including the two largest franchises in America.
See full profileUpdated June 2026
How we verify our calculators
Methodology
How the Furnace Size Calculator Works
First the calculator finds your heating load — the heat your home loses on a design-cold day — by multiplying floor area by a climate BTU/sq ft factor and an insulation adjustment. That load is the heat a furnace must deliver.
Furnaces are sold by input rating, not output, so the load is divided by the AFUE efficiency to get the input BTU to shop for, then rounded up to the next standard size. See DOE Energy Saver for efficiency guidance.
Formulas
Heating load (BTU/hr) = area × climate factor × insulation
Furnace input (BTU/hr) = heating load ÷ AFUE
Recommended size = next standard furnace ≥ input
Quick Reference
- Hot climate
- 30 BTU/sq ft
- Moderate
- 40 BTU/sq ft
- Cold climate
- 50–60 BTU/sq ft
- Standard sizes
- 40k–140k BTU
- Buy by
- input rating
Pick your efficiency
80% or high-efficiency AFUE?
AFUE sets how much of your gas becomes heat — and how big an input rating you need. The right pick depends on your climate, gas price, and how long you'll stay.
80% AFUE (standard)
non-condensing
Choose it if: Mild winters, low gas prices, a tight budget, or a home you won't keep long. Wastes 20% of the fuel up the flue.
lowest upfront cost
90–95% AFUE (condensing)
ENERGY STAR
Choose it if: Cold climates and long-term homes — the fuel savings repay the premium in a few years, and many codes now require it.
best all-round value
96–98% AFUE (premium)
modulating
Choose it if: Very cold zones with high heating bills, or when you want quiet, even heat from a variable-speed, modulating burner.
best comfort
What AFUE Costs You
To deliver the same 60,000 BTU/hr of heat, a lower-AFUE furnace must burn more fuel and carry a bigger input rating:
| AFUE | Input for 60k load |
|---|---|
| 80% | 75,000 BTU |
| 90% | 66,667 BTU |
| 95% | 63,158 BTU |
| 97% | 61,856 BTU |
The wasted share leaves as flue gas. Over a cold-climate winter that gap is real money — run the payback at your local gas price.
Furnace Sizing Examples
Cold-Climate Two-Story
2,200 sq ft · cold · 95% AFUEBig, cold-climate homes land at the top of the range. Confirm with a Manual J before buying.
Warm-Climate Ranch
1,400 sq ft · warm · 95% AFUEMild winters need surprisingly little — don't let a contractor oversize 'to be safe.'
Older, Drafty Home
1,800 sq ft · moderate · 80% AFUELeaky envelope plus a low-efficiency furnace pushes the input size up. Air-sealing first can drop you a size.
Avoid these
Furnace Sizing Mistakes
Buying by output and ignoring AFUE
Furnaces are rated by input. Divide your heating load by AFUE to get the input size to shop for.
Oversizing 'to be safe'
A too-big furnace short-cycles, wastes fuel, and heats unevenly. Right-size it.
Using a cooling BTU/sq ft rule for heating
Heating needs more BTU/sq ft than cooling in cold climates. Use a heating factor.
Skipping the Manual J on a real purchase
A square-foot estimate plans the project; a Manual J accounts for windows, ducts, and design temperature.
Furnace Size Calculator FAQs
What size furnace do I need for a 2,000 sq ft house?
How many BTUs per square foot do I need for heating?
What is AFUE and how does it affect furnace size?
Is it bad to oversize a furnace?
Should I get an 80% or a 95%+ AFUE furnace?
What's the difference between furnace input and output BTU?
Do I still need a Manual J load calculation?
Comparing heating systems? Run the numbers on a heat pump or size a room with the BTU calculator.
Important Disclaimer
These estimates are for planning purposes only. Actual costs vary by location, material availability, and project complexity. Always get at least 3 local quotes. This calculator does not replace professional advice.