HowMuchIt / Heating BTU
How many BTU do I need to heat my home?
A quick estimate starts from the heated floor area and the climate: about 30 BTU per square foot in the warmest states, up to 60 in the coldest. The calculator gives the range and the furnace size it points to.
Work out your heating BTU
| Climate | BTU per sq ft | For 1,500 sq ft |
|---|---|---|
| Warmest | 30 to 35 | 45,000 to 52,500 |
| Coldest | up to about 60 | up to about 90,000 |
Well insulated homes sit at the low end of their range, drafty ones at the high end. Furnace input = heat output / AFUE.
Output, input and AFUE
A BTU is the heat that warms one pound of water by one degree Fahrenheit. The number your home needs is heat delivered into the rooms. A furnace label shows input BTU, the fuel it burns; what reaches the house is the input times its efficiency, the AFUE. A 100,000 BTU furnace at 80 percent AFUE delivers 80,000 BTU. So compare your estimate with the output, not the big number on the label.
Why bigger is not better
A furnace that is too big heats the house in short bursts, shuts off, and starts again a few minutes later. It wastes fuel and wears out sooner, and the rooms swing from warm to cool. The per-square-foot estimate is a starting point; the real sizing is a Manual J load calculation, the method from ACCA, which a heating contractor does room by room with your windows, insulation and walls.
Questions people ask
What size furnace do I need for 1,500 square feet?
About 45,000 to 52,500 BTU of output in the warmest states, up to about 90,000 in the coldest. Insulation moves you within the range.
How many square feet does an 80,000 BTU furnace heat?
Divide its output, not its input, by your climate's figure. At 80 percent AFUE an 80,000 BTU input gives 64,000 out: about 1,070 sq ft at 60 BTU, about 2,100 at 30.
What does AFUE mean?
Annual Fuel Utilization Efficiency: the share of the fuel a furnace turns into heat for the home.
Who does a load calculation?
A heating contractor, with the Manual J method. Ask for it before you replace a furnace.
Good to know
- One BTU is the heat needed to raise one pound of water by 1 deg F.1,2,3
- Rule of thumb for heating: about 30 to 60 BTU per square foot, depending on climate and insulation.2,4
- Estimate: heated square feet times the BTU-per-square-foot factor for your climate equals the heat output needed.2,4
- Warmest climates (southern Texas, Florida, the South): about 30 to 35 BTU per square foot.2,4
- Coldest climates (northern states such as Maine): up to about 60 BTU per square foot.2,4
- A well insulated home sits at the low end of its range; a poorly insulated one at the high end.2,4
- A furnace label gives input BTU; heat output is input times the efficiency (AFUE): 100,000 BTU in at 80 percent gives 80,000 BTU out.2,4,5
- AFUE (Annual Fuel Utilization Efficiency) is the share of the fuel a furnace turns into heat for the home.4,5,6
- The real sizing method is a Manual J load calculation (from ACCA), done by a contractor; a per-square-foot estimate is only a starting point.3,4,7
- Bigger is not better: an oversized furnace turns on and off in short bursts, wasting energy and wearing out sooner.2,3
How we checked this page
Checked on October 27, 2026. We kept a fact only when two independent publishers agree, and each one is numbered to its sources below. Where sources give different numbers, we keep only what they share. We never write a price. This page was written with an AI assistant and checked against the sources below.
Sources
- British thermal units (Btu), U.S. Energy Information Administration
- Solved! What Size Furnace Do I Need? How To Calculate Furnace Size, Bob Vila
- What Size Furnace Do I Need? A Guide to Furnace Sizing, HVAC.com
- What Size Furnace Do I Need? A Homeowner's Guide to BTU Sizing, Bryant (Carrier)
- What Does AFUE Stand For?, Trane
- Furnaces Key Product Criteria, ENERGY STAR
- Who Can Perform My Load Calculations?, GreenBuildingAdvisor (Taunton)