Estimate the furnace size your home needs from square footage, climate zone, and insulation — with AFUE input/output correction, standard-size matching, and an oversize checker.
This tool gives a planning estimate, not a professional sizing. The industry standard for final furnace selection is an ACCA Manual J residential load calculation performed by a qualified HVAC contractor. Use this calculator to sanity-check quotes and budget — always get a Manual J before you buy.
All heated areas only — exclude unheated basements and garages.
Zone factors from the published PickHVAC sizing table.
Estimates assume 8-ft ceilings as the baseline.
80 = standard, 90–98 = high-efficiency condensing.
Standard residential furnaces come in ~20,000 BTU/hr steps, so the recommended size is rounded up — a small oversize is normal and safer than undersizing. Still, confirm with an ACCA Manual J load calculation before purchasing.
Already have a furnace (or a quote)? Check whether its output matches your home's heating load. An oversized furnace short-cycles — it heats too fast, shuts off, and restarts — causing uneven temperatures, extra wear, and higher bills.
Tip: run the Size Calculator first, then click “use my result”.
Quick reference: estimated heating load in BTU/hr output by home size and climate zone (average insulation, 8-ft ceilings). Divide by your AFUE to get the input rating to shop for.
| Sq ft | Z1 30 | Z2 35 | Z3 40 | Z4 45 | Z5 50 | Z6 55 | Z7 60 |
|---|
Column headers show the BTU-per-square-foot factor for each zone.
Compare running cost per unit of delivered heat. Gas furnaces lose a share of fuel energy up the flue (that's the AFUE); electric resistance heat converts ~100% of electricity to heat but electricity usually costs more per BTU.
Optional — leave blank for cost per MMBTU only.
Heating load (output BTU/hr) = square footage × zone factor × insulation × ceiling × windows × sunlight.
| Zone | Factor | Description |
|---|---|---|
| Zone 1 | 30 BTU/sq ft | Warmest — e.g. Miami |
| Zone 2 | 35 BTU/sq ft | Warm |
| Zone 3 | 40 BTU/sq ft | Moderate-warm — e.g. Memphis, TN |
| Zone 4 | 45 BTU/sq ft | Moderate |
| Zone 5 | 50 BTU/sq ft | Cool |
| Zone 6 | 55 BTU/sq ft | Cold |
| Zone 7 | 60 BTU/sq ft | Coldest — northern tier |
Adjustment factors (estimator assumptions, applied on top of the published zone table): insulation good −10% / average ±0% / poor +10%; ceiling height scaled against the 8-ft baseline; many/large windows +5%; sunny lot −3% / heavily shaded +3%.
AFUE correction: furnace input rating = heating load ÷ AFUE, and furnace output = input × AFUE. Manufacturers rate furnaces by input; your home feels the output.
Standard sizes: most residential gas furnaces are sold from 40,000 to 140,000 BTU/hr input in roughly 20,000 BTU/hr steps. The tool recommends the smallest standard size at or above your required input.
Sources & method notes: zone factors, the Input × AFUE = Output equation, the 40,000–140,000 BTU/hr residential range with 10,000–20,000 BTU/hr steps, and the “round slightly up” guidance are from PickHVAC’s published sizing guide (pickhvac.com/furnace/sizing/). The 8-ft ceiling baseline matches the industry-standard estimate assumption; the insulation range concept (“use the lower end of the range if well insulated, the higher end if poorly insulated”) follows the published EIA-zone guidance. Oversize verdict bands and cost examples are this tool’s disclosed estimator logic — not a substitute for an ACCA Manual J calculation by a qualified contractor.
It depends on your climate zone. Heating load = square footage x zone factor: Zone 1 (30 BTU/sq ft) gives 60,000 BTU/hr output; Zone 4 (45) gives 90,000 BTU/hr; Zone 7 (60) gives 120,000 BTU/hr. Divide by AFUE for the input rating: at 80% AFUE that is 75,000, 112,500, and 150,000 BTU/hr input — so you would buy the 80,000 or 120,000 BTU/hr standard size. A 150,000 BTU/hr load exceeds the largest standard residential size (140,000 BTU/hr), so get a professional heat-loss calculation.
Output = Input x AFUE. A 100,000 BTU/hr furnace at 95% AFUE delivers 95,000 BTU/hr of heat. In reverse: if your home needs 90,000 BTU/hr and you are buying an 80% AFUE furnace, you need 90,000 / 0.80 = 112,500 BTU/hr input — so the 120,000 BTU/hr model.
Most brands build gas furnaces from 40,000 to 140,000 BTU/hr input in roughly 20,000 BTU/hr steps: 40k, 60k, 80k, 100k, 120k, 140k. Because of these steps, round your calculated input UP to the next size — a slightly larger furnace is safer than one that is slightly too small.
An oversized furnace short-cycles: it heats too fast, shuts off, and restarts — causing uneven temperatures, extra wear, and higher bills. Example: your home needs 60,000 BTU/hr of output, but you install a 120,000 BTU/hr, 80% AFUE furnace (96,000 BTU/hr output) — that is 60.0% oversized. Aim to land within about 15-30% above your load; the 20,000 BTU/hr size steps make a small oversize unavoidable.
It is a planning estimate, not a design calculation. A square-foot rule cannot see your ductwork, air leakage, window area, or exact winter design temperature. The industry standard for final sizing is the ACCA Manual J residential load calculation — always get one before you buy. Use this tool to sanity-check contractor quotes and to budget.
Compare cost per MMBTU of delivered heat. Example: gas at $1.50/therm with a 95% AFUE furnace costs $1.50 / (0.1 x 0.95) = $15.79 per MMBTU; electricity at $0.17/kWh costs $0.17 / 0.003412 = $49.82 per MMBTU (electric resistance is about 100% efficient). For a home using 60 MMBTU of heat per year, that is $947.37 vs $2,989.45. Enter your own rates in the cost tab — prices vary widely.
This calculator uses the 7-zone scheme from PickHVAC's published sizing guide: Zone 1 is warmest (for example Miami), Zone 3 covers areas like Memphis, TN, and Zone 7 is the coldest northern tier. If you live near a zone boundary, pick the warmer zone if you keep the thermostat high in winter, or the colder zone if you keep it low.
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