Furnace Size Calculator
Enter your heated floor area, climate zone and insulation level, plus the efficiency of the furnace you are looking at. You get the heat loss to cover and the furnace input size to buy.
- US climate zones 1 to 7
- AFUE efficiency built in
- Standard furnace sizes
A quick estimate for planning. Contractors use an ACCA Manual J calculation for the final size.
Furnaces are sold by input, homes lose heat by output. A 80,000 BTU furnace with 95% AFUE delivers 76,000 BTU of heat. One at 80% AFUE delivers only 64,000. Always compare the output to your home's heat loss, then choose the input size that gets you there.
Furnace size by home and zone
Input BTU for a 95% AFUE furnace. Search for your square footage or zone.
| Home (sq ft) | Zone 2 | Zone 4 | Zone 5 | Zone 6 |
|---|---|---|---|---|
| 1,000 | 39,000 | 49,000 | 55,000 | 60,000 |
| 1,200 | 47,000 | 59,000 | 66,000 | 72,000 |
| 1,500 | 58,000 | 74,000 | 82,000 | 90,000 |
| 1,800 | 70,000 | 89,000 | 99,000 | 108,000 |
| 2,000 | 78,000 | 99,000 | 109,000 | 120,000 |
| 2,500 | 97,000 | 124,000 | 137,000 | 150,000 |
| 3,000 | 117,000 | 148,000 | 164,000 | 180,000 |
Figures assume average insulation and 8 ft ceilings. Well-sealed modern homes can need 20% less, and drafty older homes 15% to 25% more.
Climate zone changes everything
The same house in four zones. Slide the home size to compare heat loads.
The Department of Energy climate zones follow winter severity. Zone 7 needs almost twice the heat per square foot of zone 1.
How to estimate furnace size
Five steps from floor area to furnace. Follow along with your own numbers.
Measure heated floor area
Count every room you heat, not the garage or unheated basement.
If your old furnace never struggled on the coldest days and ran in long cycles, its output is a useful real-world check.
80% or 95% AFUE
Efficiency changes the size you buy. Compare the two common furnace types.
Vents through a metal chimney. Lower price, but 20% of the fuel heats the outdoors.
Condensing furnace that vents through plastic pipe. More heat from every unit of gas.
Moves heat rather than burning fuel. Cold-climate models work well below freezing.
In cold climates the higher upfront cost of a 95% furnace usually pays back within a few years. In mild climates, an 80% unit can still make sense.
Before replacing a furnace
Questions to ask your contractor. Tick each one.
Tick each item as you go.
Oversizing is the most common mistake. A furnace that is too large heats in short bursts, which wastes fuel, wears out parts and leaves cold spots.
What you get from the result
Heat loss, input and output. Each tile explains one result.
BTU per hour your home loses on a design cold day.
Heat loss divided by AFUE.
The next standard input rating.
For comparing with electric or heat pump units.
A quick check against rules of thumb.
Furnace nameplates list both input and output. Compare the output to the heat loss figure here to be sure it can keep up on the coldest night.
Frequently Asked Questions
No question matches that search.
What size furnace do I need for 2,000 sq ft?
About 80,000 to 110,000 BTU input in zones 4 and 5 with a 95% furnace. Milder zones need less.
How many BTU per square foot for heating?
About 30 to 35 in warm climates, 45 to 50 in mixed climates and 55 to 60 or more in cold climates.
Is a bigger furnace better?
No. An oversized furnace short cycles, wastes fuel and wears out faster. Aim for no more than 25% above the heat loss.
What does AFUE mean?
Annual fuel utilisation efficiency, the share of fuel energy that becomes heat in your home over a year.
What size furnace for 1,500 sq ft?
About 60,000 to 80,000 BTU input in a mixed climate with a high efficiency furnace.
Do ceiling heights matter?
Yes. Taller ceilings mean more air to heat. The calculator scales the load for ceilings above 8 ft.
Should I size a furnace for the basement?
Only if the basement is finished and heated. Unheated basements are left out of the area.
Heat the home, not the chimney
Measure your heated area, pick your climate zone and insulation honestly and divide by the furnace efficiency. Use the result to check contractor quotes, and ask for a Manual J if the numbers are far apart.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.