Two different BTU numbers, and the sticker shows one of them
Almost every furnace-sizing mistake starts with one fact: a furnace nameplate is rated in input BTU, which is how much gas the burner consumes per hour. The heat that actually reaches your rooms is output BTU, which is input multiplied by the furnace's AFUE efficiency. Those are different numbers, and quotes almost always mean input.
- Heating load (output) — what the house loses on a design day, so what must be delivered. This is what sizing rules actually estimate.
- Input BTU — load ÷ AFUE. This is the number a furnace is sold by, and the number to compare between quotes.
At 95% AFUE, a 100,000 BTU/h input furnace delivers about 95,000 BTU/h. At 80% AFUE, the same nameplate delivers 80,000. Same sticker, meaningfully different heat.
One unit never to use here: the ton. A ton is a cooling unit equal to 12,000 BTU/h, and it appears on air conditioners and heat pumps. Furnaces are sized in BTU/h. If a heating quote is described in tons, something is being confused.
The formula behind the answer
Three steps, and skipping any one of them is where estimates go wrong:
- Heat load:
load (BTU/h) = floor area (sq ft) × climate factor - Convert to the rating you buy:
input (BTU/h) = load ÷ AFUE - Pick the size: round up to the next standard furnace class: 40,000 / 60,000 / 80,000 / 100,000 / 120,000 / 140,000 BTU/h input
The climate factor is a stand-in for heating degree days — how cold it gets and for how long. Two identical houses can differ by three or four times in annual heating demand purely because of location, which is why "BTU per square foot" is meaningless without saying where.
| Climate factor | Where it applies | Example: 2,000 sq ft |
|---|---|---|
| 30 BTU/sq ft — warm | Dallas, Houston, Atlanta, Phoenix | 60,000 BTU/h load |
| 40 BTU/sq ft — moderate | New York, St. Louis, Washington DC, Kansas City | 80,000 BTU/h load |
| 50 BTU/sq ft — cold | Chicago, Boston, Minneapolis, Denver, Cleveland | 100,000 BTU/h load |
| 60 BTU/sq ft — very cold | Duluth, Fargo, Anchorage | 120,000 BTU/h load |
These factors assume average residential construction: 2×6 walls around R-19, R-30 to R-49 ceilings, double-pane low-E windows. Pre-1980 houses with R-11 walls and single glazing should be pushed up 10–20%; a passive or super-tight build comes down 30–40%.
Three worked examples
- 2,000 sq ft, moderate climate, 95% AFUE. Load = 2,000 × 40 = 80,000 BTU/h. Input = 80,000 ÷ 0.95 = 84,211 BTU/h, which rounds up to the next standard class: a 100,000 BTU/h furnace.
- 1,500 sq ft, cold climate, 80% AFUE. Load = 1,500 × 50 = 75,000 BTU/h. Input = 75,000 ÷ 0.80 = 93,750 BTU/h → 100,000 BTU/h furnace. Note how the lower efficiency eats most of the headroom here — at 95% the same house would have come in at 78,947, one class down.
- 2,500 sq ft, very cold climate, 80% AFUE. Load = 2,500 × 60 = 150,000 BTU/h. Input = 150,000 ÷ 0.80 = 187,500 BTU/h, which is past the largest single residential class. The right answer is not one giant furnace — it is two zones with smaller furnaces. Single-stage units above 120,000 also need gas line pressure that many residential meters cannot supply.
What a square-foot rule of thumb cannot see
Area and climate get you into the right neighbourhood. A real Manual J load calculation — the standard method published by ACCA — also accounts for: window area, orientation and glazing type; insulation levels in walls, ceiling and floor; air infiltration, which is often the largest single term in an older house; ceiling height above the 8 ft these factors assume; duct losses if ducts run through unconditioned space; and the number of occupants.
Treat the calculator as a sanity check on a contractor's quote, and as a way to catch an obvious mismatch. If a quote is two classes away from what square footage and your climate suggest, ask which line of the load calculation explains it.
The oversizing trap
Bigger is not safer. An oversized furnace reaches setpoint in a few minutes, shuts off, then repeats — short cycling. That wastes fuel, wears the igniter and blower, and leaves rooms uneven because the air never mixes. Undersizing fails differently: the furnace simply runs continuously and cannot hold temperature on the coldest days.
Equipment type changes how much this matters. A single-stage furnace has one speed and is least forgiving. Two-stage units run near 65% capacity most of the time. Modulating furnaces track demand continuously between roughly 35% and 100%, and are the most tolerant of a slightly generous size.
Common mistakes
- Comparing an input rating against an output rating. Convert first, or the comparison is meaningless.
- Describing a furnace in tons. Tons are cooling. Furnaces are BTU/h.
- Rounding down to the nearest size. Furnaces come in fixed classes; rounding down leaves you short on the design day.
- Using one climate factor for the whole country. The spread is 30 to 60 BTU/sq ft — a factor of two.
- Ignoring ceiling height. The factors assume 8 ft. A 12 ft room adds roughly half again as much volume to heat.
- Sizing from the old furnace's nameplate. The previous unit may have been wrong, or sized for a house that has since been insulated.
- Not checking the gas service. A large single-stage furnace can exceed what an existing meter and line can deliver.
What to check once the size is settled
Sizing decides the furnace; these decide whether it actually runs well. All four are bought after the load calculation, and two of them are safety items rather than conveniences.
- Programmable thermostat — Pairing a correctly sized furnace with a decent thermostat is what stops short cycling from turning into temperature swings. Smart models also log run time, which shows whether your sizing guess held up.
- Furnace air filter (check the size) — The cheapest maintenance item and the most common cause of low airflow and high temperature rise. Buy the size printed on the rack, not the one you think you have.
- Carbon monoxide alarm — Non-negotiable with any fuel-burning appliance. Put one near sleeping areas and one on each level, and check the sensor date.
- Duct insulation and foil tape — If ducts run through an unconditioned space, sealing and insulating them is usually cheaper than the next furnace size up.
As an Amazon Associate I earn from qualifying purchases. The links above are affiliate links — if you buy through one, the price you pay is unchanged and a small commission helps keep these calculators free. Tools are listed because the job calls for them, not because of commission rates.
Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on the equipment manufacturer, the page says so instead of guessing one.
- ACCA (Air Conditioning Contractors of America) — Manual J residential load calculation and Manual S equipment selection standards.
- U.S. Department of Energy — furnace efficiency (AFUE), heating system sizing and home energy guidance.
- ASHRAE — climate design conditions and HVAC engineering practice.
Figures, formulas and reference tables last verified: 21 September 2026.