Furnace & heating BTU calculator
Short answer
The furnace size you need depends on your home’s area and climate. A 1,500 sq ft home needs roughly 45,000–75,000 BTU of heat output — about 63,000 in a temperate climate — which is a 66,000–80,000 BTU input furnace at 95% AFUE. Colder climates and poorer insulation push it higher. Enter your square footage and climate for a specific estimate.
Estimates the heat output your home needs and the furnace input size to supply it at that efficiency. A rough sizing guide — a pro should confirm with a Manual J load calculation.
Furnace size by house size
Heat output (BTU/hr) needed by floor area and climate, at average insulation and an 8-ft ceiling.
| Home size | Warm | Temperate | Cold |
|---|---|---|---|
| 800 sq ft | 30,000 BTU | 34,000 BTU | 44,000 BTU |
| 1,000 sq ft | 37,000 BTU | 42,000 BTU | 55,000 BTU |
| 1,200 sq ft | 44,000 BTU | 50,000 BTU | 66,000 BTU |
| 1,500 sq ft | 56,000 BTU | 63,000 BTU | 83,000 BTU |
| 2,000 sq ft | 74,000 BTU | 84,000 BTU | 110,000 BTU |
| 2,500 sq ft | 93,000 BTU | 105,000 BTU | 138,000 BTU |
How to size a furnace
Furnace sizing starts with your home’s heating load: the BTU of heat per hour it loses on a cold day. A quick estimate multiplies your floor area by a climate factor — roughly 30–35 BTU per sq ft in warm regions up to 50–60 in cold ones — then adjusts for insulation and ceiling height. That gives the output your furnace must deliver.
Because furnaces are sold by fuel input, divide the output by the unit’s AFUE efficiency to get the input size to shop for, then round up to the nearest standard furnace (40k, 60k, 80k, 100k, 120k BTU). Avoid oversizing: a furnace that’s too big short-cycles and heats unevenly. For a final spec, an installer should run a Manual J load calculation.
Sizing your AC too?
The main calculator sizes cooling BTU, tonnage, and the unit type to buy.
Open the BTU / AC calculatorFrequently asked questions
What size furnace do I need?
It depends on the area you’re heating and your climate. A 1,500 sq ft home needs about 63,000 BTU of heat output in a temperate climate — more in a cold climate or a poorly insulated house, less in a warm or well-sealed one. That output maps to roughly a 66,000–80,000 BTU input furnace at 95% AFUE. Enter your details above for a specific number.
How many BTUs do I need to heat a house?
Multiply your floor area by a climate factor: about 30–35 BTU per sq ft in warm zones, 40–45 in temperate zones, and 50–60 in cold zones. A 2,000 sq ft home in a cold climate needs roughly 110,000 BTU of heat output; the same home in a warm climate needs about 64,000.
What is the difference between furnace input and output BTU?
Furnaces are rated by fuel input, but you heat your home with the output. Output = input × AFUE. An 80,000 BTU input furnace at 95% AFUE delivers about 76,000 BTU of usable heat. Size the furnace so its output covers your home’s heating load.
What is AFUE?
AFUE (Annual Fuel Utilization Efficiency) is the percentage of fuel a furnace turns into heat over a year. An 80% AFUE furnace wastes 20% up the flue; high-efficiency condensing furnaces reach 95–98%. Higher AFUE means a smaller input furnace delivers the same heat.
Is it bad to oversize a furnace?
Yes. An oversized furnace short-cycles — it heats quickly, shuts off, and repeats — which wastes fuel, wears the equipment, and leaves uneven temperatures and humidity. Size to the actual load with only a small buffer for the coldest days rather than guessing big.