Furnace Size Calculator (BTU Input from Heat Loss and AFUE)
Turn a design heat loss into the furnace you should shop for: required heat output with a sizing margin, the nameplate input BTU/h at your AFUE, the nearest standard furnace size and its real output. Enter the heat loss directly or estimate it from floor area and climate zone.
Last updated: October 2026
Required Furnace Input
52,105 BTU/h
Formula below · 2 sources (energystar.gov, acca.org) · Updated Oct 2026
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About this calculator
Furnaces are sold by input: the BTU per hour of gas or oil they burn at full fire, printed on the nameplate as 40,000, 60,000, 80,000 and so on. What heats the house is the output, input × AFUE, where the annual fuel utilization efficiency is the share of the fuel's energy that reaches the supply air (80% for a non-condensing furnace, 90-98% for a condensing one; ENERGY STAR requires 95% in the US South and 97% in the North). So a furnace is sized in three steps: find the design heat loss, add a margin, and divide by the AFUE to get the input to shop for.
The heat loss
The right starting point is a surface-by-surface calculation (the heat loss calculator, or a full ACCA Manual J): walls, ceiling, floor and windows at U × A × ΔT plus infiltration, at the 99% winter design temperature. If you only have a floor area, the second mode multiplies it by a climate rule of thumb, 30 BTU/h per square foot in hot climates up to 60 in very cold ones. That is a budgeting figure for an averagely insulated house; a tight new house can be half of it and an old leaky one well above, which is why the rule is marked rough.
The margin
ACCA Manual S allows a furnace output of up to 140% of the heating load, and most installers pick the smallest size that clears the load with 10-25% to spare for pick-up after a setback and for colder-than-design nights. Bigger is not better: an oversized furnace reaches the thermostat setting in a few minutes and shuts off, so it short-cycles, wears its igniter and blower, moves air that never warms properly and delivers less of its rated efficiency.
Standard sizes
Residential gas furnaces come in 20,000 BTU/h steps from about 40,000 to 120,000-140,000 input. The calculator rounds your required input up to the next step and shows what that furnace actually outputs and how far above the heat loss it lands. A two-stage or modulating furnace at the next size up is a good way to keep a margin without short-cycling, since it runs at 40-65% of capacity most of the time.
Output of standard furnace input sizes at common AFUE ratings (BTU/h)
Scroll sideways to see all columns →
| Nameplate input | 80% AFUE | 90% AFUE | 95% AFUE | 97% AFUE |
|---|---|---|---|---|
| 40,000 | 32,000 | 36,000 | 38,000 | 38,800 |
| 60,000 | 48,000 | 54,000 | 57,000 | 58,200 |
| 80,000 | 64,000 | 72,000 | 76,000 | 77,600 |
| 100,000 | 80,000 | 90,000 | 95,000 | 97,000 |
| 120,000 | 96,000 | 108,000 | 114,000 | 116,400 |
How to use
Example: a heat loss calculation gives 45,000 BTU/h for the house. With a 10% margin the furnace must put out 45,000 × 1.10 = 49,500 BTU/h. At 95% AFUE that needs an input of 49,500 ÷ 0.95 = 52,105 BTU/h, so the furnace to buy is the next standard size, a 60,000 BTU/h input unit, which delivers 60,000 × 0.95 = 57,000 BTU/h, 27% above the heat loss. Those are the defaults. Second example, area mode: a 2,000 sq ft house in a cool climate (zone 5, 50 BTU/h per sq ft) is estimated at 100,000 BTU/h of loss; with a 20% margin and an 80% furnace the input is 100,000 × 1.2 ÷ 0.8 = 150,000 BTU/h, which is beyond a single ordinary residential furnace and a sign that the rough rule is overstating a reasonably insulated house: run the surface-by-surface calculation instead.
Frequently asked questions
What size furnace do I need for 2,000 square feet?
It depends far more on climate and insulation than on the area. The rule of thumb runs from 30 BTU/h per square foot in a hot climate (60,000 BTU/h of heat loss) to 60 in a very cold one (120,000). Divide by the AFUE for the input: at 95%, 60,000 of loss with a 10% margin needs a 70,000 BTU/h input, so an 80,000 furnace. A surface-by-surface heat loss calculation will usually come out lower than the rule for a house built to recent codes.
What is AFUE and why divide by it?
AFUE, annual fuel utilization efficiency, is the fraction of the fuel energy a furnace turns into heat delivered to the ducts over a heating season. Nameplates quote input (fuel burned), so output = input × AFUE. To get an output of 49,500 BTU/h from a 95% furnace you need 49,500 ÷ 0.95 = 52,105 BTU/h of input. An 80% furnace would need 61,875.
Is it better to oversize a furnace?
No. ACCA Manual S caps furnace output at 140% of the heating load, and the usual practice is 10-25% over. More than that and the furnace short-cycles: it heats the air quickly, trips the thermostat and shuts down before the ducts and rooms warm evenly, which wastes fuel, wears parts and makes comfort worse. If you are between sizes, a two-stage or modulating furnace at the larger size runs at low fire most of the time and avoids the problem.
How do I find the design heat loss?
Add up U × A × ΔT for each exterior surface plus infiltration at your 99% winter design temperature; the heat loss calculator does this from R-values and areas, and ACCA Manual J is the full version an HVAC contractor should run. The heat load calculator gives a quicker whole-house estimate from floor area, ceiling height, insulation level and temperature difference.
What are the standard furnace sizes?
Residential gas furnaces are made in roughly 20,000 BTU/h input steps: 40,000, 60,000, 80,000, 100,000, 120,000 and sometimes 140,000. Some makers offer 45,000, 70,000 or 90,000 models. The calculator rounds up to the 20,000 step; if a nearby in-between size exists it may fit better, so check the output (input × AFUE) against the required output.
Does this work for a boiler or heat pump?
The required output applies to any heating appliance. Boilers are also rated by input and AFUE, so the input figure carries over. Heat pumps are rated by output capacity at a given outdoor temperature and lose capacity as it gets colder, so size them from the output figure and the manufacturer's capacity table at your design temperature, not from this calculator's input figure.
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Sources & references
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