Furnace Size Calculator — Input & Output BTU
Estimate the heating output your home needs from area and climate, then the input (firing) rate implied by the furnace’s AFUE efficiency. A planning estimate only — gas work belongs to a licensed pro.
Calculator
A 1,500 sq ft home needs about 60,000 BTU/h of output; at 80% AFUE that is roughly 75,000 BTU/h of input. A load calc and a licensed installer are required before buying.
Furnaces are rated two ways and the difference trips people up. Output (also called capacity) is the heat actually delivered to the house. Input is the fuel the burner consumes. The gap between them is efficiency — the AFUE (Annual Fuel Utilization Efficiency). An 80% furnace turns 80% of its input into delivered heat and sends the rest up the flue; a 96% condensing furnace wastes far less.
This tool estimates the output your climate and floor area call for, then works backward through AFUE to the input (firing) rate you would see on a nameplate. That lets you compare a nameplate “100,000 BTU” furnace against what your house actually needs.
Formula
Two steps, output then input:
output_BTU = area × heating_band (30–60 BTU/sq ft typical)\ninput_BTU = output_BTU / AFUE (AFUE as a fraction, e.g. 0.80)
The heating band (about 30 BTU/sq ft in a mild winter up to 60 in a very cold one) is a labeled climate typical, not a measured design load. AFUE test basis: DOE. See Sources.
Worked example
A 1,500 sq ft home in a moderate winter climate (40 BTU/sq ft) with a standard 80% furnace:
output = 1,500 × 40 = 60,000 BTU/h delivered\ninput = 60,000 / 0.80 = 75,000 BTU/h fired
So the house needs about 60,000 BTU/h of output, which an 80% furnace delivers by firing 75,000 BTU/h of fuel. Swap in a 96% condensing furnace and the same 60,000 BTU output only needs about 62,500 BTU of input — the efficiency difference you pay for on the gas bill.
Combustion safety & sizing
Combustion safety. Gas and oil furnaces produce carbon monoxide when they burn fuel. Sizing, venting, gas piping and combustion setup are the work of a licensed installer, and every home with fuel-burning equipment should have working CO alarms. This calculator is for planning estimates only.
Do not just match the old furnace. Older furnaces were routinely oversized, so copying the nameplate perpetuates the mistake. An oversized furnace short-cycles, produces uneven temperatures and stresses the heat exchanger. A Manual J load calc, not a rule of thumb, is the correct basis for a purchase.
Reference table
| System type | Typical AFUE |
|---|---|
| Old / non-condensing gas | 0.56–0.70 |
| Standard gas | 0.80–0.83 |
| Condensing gas | 0.90–0.98 |
| Oil furnace | 0.80–0.90 |
| Electric resistance | 1.00 |
Labeled typical bands, not a spec sheet. Enter your unit’s real AFUE for the input estimate.
Frequently asked questions
What is the difference between input and output BTU?
Input is the fuel the burner consumes; output is the heat actually delivered to your home. Output = input × AFUE. Size your house on output, then check the input on the nameplate.
What size furnace do I need for 1,500 sq ft?
In a moderate climate a 1,500 sq ft home needs roughly 60,000 BTU/h of output. At 80% AFUE that is about a 75,000 BTU/h input furnace; at 96% AFUE, about 62,500 BTU/h input.
Does a higher-AFUE furnace need a bigger input rating?
No — the opposite. For the same delivered output, a higher AFUE means a lower input, because less heat goes up the flue. That lower input is what saves fuel.
Can I install a gas furnace myself?
No. Gas piping, venting and combustion setup must be done by a licensed professional, and improper work risks fire and carbon-monoxide poisoning. Use this tool for planning only, and install CO alarms.