Furnace Size Calculator (BTU by Climate Zone)
Enter your square footage, insulation and the furnace's AFUE, pick your state, and get the output your house needs and the input rating to look for on the furnace.
Furnace size
B07Scenario A
Sets the electricity rate. Pick US average if you're not sure.
From EIA for US average. Use the rate on your bill if you know it.
Rate used: 18.4¢/kWh · US average · EIA reference 2025–26 · design temp 10 °F · climate zone 4 (data as of 2026-09-26) ·
Furnace size to shop for
4.0 tons of heating at 10 °F outside (US average 99% design temperature). Confirm with a Manual J load calculation before you buy.
- Design temperature difference
- 60 °F (indoor 70 °F − US average design 10 °F)
- Heat loss at design temperature
- 42,000 BTU/h
- Heating capacity with 15% margin
- 48,300 BTU/h
- Size in tons
- 4.0 tons
- Furnace input rating (÷ AFUE)
- 50,842 BTU/h
- Furnace size to shop for (rounded up)
- 60,000 BTU/h
Estimate. Formula and sources are under “How it works”. Change any input; the result updates as you type.
2026 note: The federal 25C and 25D tax credits ended for installations after Dec 31, 2025. Our numbers use 2026 rules. State rebates, HEAR/HOMES programs and utility programs may still apply.
Formula
How it works
- Design temperature difference = indoor temperature − your state's 99% winter design temperature
- Heat loss (BTU/h) = square feet × insulation factor × temperature difference (good 0.25, average 0.35, poor 0.50 BTU/h·°F per sq ft)
- Output needed = heat loss × 1.15 (15% margin; ACCA Manual S limits heating oversizing to 140%)
- Furnace input = output needed ÷ AFUE
Furnaces are sold by input rating, the gas they burn per hour. The heat they deliver is input × AFUE. So an 80% furnace needs a bigger input number than a 96% furnace for the same house. Residential furnaces come in steps of about 20,000 BTU (40,000, 60,000, 80,000, 100,000, 120,000); pick the first size at or above the input rating shown.
Sources: ASHRAE Handbook of Fundamentals and ACCA Manual J table 1A (99% design temperatures, each state's most populous city); ACCA Manual J (simplified envelope factors); ACCA Manual S (oversizing limit).
Worked example
A worked example
With Texas's 25 °F design temperature, the difference is 45 °F (indoor 70 − design 25). A well-insulated 2,500 sq ft house loses 28,125 BTU/h. With the 15% margin it needs 32,344 BTU/h of output (2.7 ton). A 95% furnace needs an input rating of at least 34,046 BTU/h, so the smallest common size, 40,000 BTU, covers it.
Reference
By state, at the default inputs
Furnace size to shop for, with this page's default inputs and each state's reference prices (EIA reference 2025–26, data as of 2026-09-26).
| US average | 60,000 BTU/h input | 18.4¢/kWh |
|---|---|---|
| Alabama | 60,000 BTU/h input | 15.5¢/kWh |
| Alaska | 80,000 BTU/h input | 25.0¢/kWh |
| Arizona | 40,000 BTU/h input | 15.0¢/kWh |
| Arkansas | 60,000 BTU/h input | 13.0¢/kWh |
| California | 40,000 BTU/h input | 32.0¢/kWh |
| Colorado | 60,000 BTU/h input | 16.0¢/kWh |
| Connecticut | 60,000 BTU/h input | 30.0¢/kWh |
| Delaware | 60,000 BTU/h input | 17.0¢/kWh |
| District of Columbia | 60,000 BTU/h input | 19.0¢/kWh |
| Florida | 40,000 BTU/h input | 15.0¢/kWh |
| Georgia | 60,000 BTU/h input | 14.5¢/kWh |
| Hawaii | 40,000 BTU/h input | 42.0¢/kWh |
| Idaho | 60,000 BTU/h input | 12.0¢/kWh |
| Illinois | 80,000 BTU/h input | 17.0¢/kWh |
| Indiana | 60,000 BTU/h input | 15.5¢/kWh |
| Iowa | 80,000 BTU/h input | 13.5¢/kWh |
| Kansas | 60,000 BTU/h input | 14.5¢/kWh |
| Kentucky | 60,000 BTU/h input | 13.0¢/kWh |
| Louisiana | 40,000 BTU/h input | 12.5¢/kWh |
| Maine | 80,000 BTU/h input | 26.0¢/kWh |
| Maryland | 60,000 BTU/h input | 19.0¢/kWh |
| Massachusetts | 60,000 BTU/h input | 29.0¢/kWh |
| Michigan | 60,000 BTU/h input | 19.5¢/kWh |
| Minnesota | 80,000 BTU/h input | 15.5¢/kWh |
| Mississippi | 60,000 BTU/h input | 13.5¢/kWh |
| Missouri | 60,000 BTU/h input | 13.0¢/kWh |
| Montana | 80,000 BTU/h input | 13.0¢/kWh |
| Nebraska | 80,000 BTU/h input | 12.0¢/kWh |
| Nevada | 60,000 BTU/h input | 15.0¢/kWh |
| New Hampshire | 80,000 BTU/h input | 27.0¢/kWh |
| New Jersey | 60,000 BTU/h input | 22.0¢/kWh |
| New Mexico | 60,000 BTU/h input | 15.0¢/kWh |
| New York | 60,000 BTU/h input | 25.0¢/kWh |
| North Carolina | 60,000 BTU/h input | 14.5¢/kWh |
| North Dakota | 80,000 BTU/h input | 11.5¢/kWh |
| Ohio | 60,000 BTU/h input | 16.5¢/kWh |
| Oklahoma | 60,000 BTU/h input | 13.0¢/kWh |
| Oregon | 60,000 BTU/h input | 14.5¢/kWh |
| Pennsylvania | 60,000 BTU/h input | 18.5¢/kWh |
| Rhode Island | 60,000 BTU/h input | 29.0¢/kWh |
| South Carolina | 60,000 BTU/h input | 15.0¢/kWh |
| South Dakota | 80,000 BTU/h input | 13.0¢/kWh |
| Tennessee | 60,000 BTU/h input | 13.5¢/kWh |
| Texas | 40,000 BTU/h input | 15.5¢/kWh |
| Utah | 60,000 BTU/h input | 12.0¢/kWh |
| Vermont | 80,000 BTU/h input | 22.0¢/kWh |
| Virginia | 60,000 BTU/h input | 15.5¢/kWh |
| Washington | 60,000 BTU/h input | 12.0¢/kWh |
| West Virginia | 60,000 BTU/h input | 15.5¢/kWh |
| Wisconsin | 80,000 BTU/h input | 17.5¢/kWh |
| Wyoming | 80,000 BTU/h input | 12.5¢/kWh |
Questions
FAQ
What size furnace do I need for 1,500 sq ft?
At the US-average design temperature (10 °F) with average insulation: heat loss 31,500 BTU/h, output with margin 36,225 BTU/h. A 95% furnace needs an input of 38,132 BTU/h, so a 40,000 BTU furnace. An 80% furnace needs 45,281 BTU/h, so a 60,000 BTU furnace.
80% vs 96% AFUE: does the size change?
The heat your house needs doesn't change; the input rating does. Input = output ÷ AFUE, so an 80% furnace needs about 20% more input than a 96% one for the same output.
Is a bigger furnace better?
No. An oversized furnace heats in short bursts, cycles on and off more, and can make rooms uneven. The calculator adds a 15% margin to the design heat loss, not more.
Why does my old furnace have a much bigger rating?
Many older furnaces were sized with rules of thumb per square foot, before the house got new windows or attic insulation. If yours is far above the number here, ask the installer to run Manual J before you replace it like-for-like.
What design temperature does the calculator use for my state?
The 99% heating design temperature of the state's most populous city: for example −5 °F in Illinois, 25 °F in Texas, −15 °F in Minnesota. If you live far from that city, your installer's Manual J will use your county's value.
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