~46,000 BTU output / 50,000 BTU input @ 96%
Heating Output
46,000
BTU/hr
Recommended: 60 kBTU furnace
96% AFUE → 57,600 BTU output, covers your 46,000 BTU load (~25% margin)
Input @ 80% AFUE
60,000
BTU/hr
Input @ 96% AFUE
50,000
BTU/hr
Recommended efficiency
96% AFUE
Larger home in moderate climate — 96% AFUE pays back over time.
How we calculated this
= 45,000 BTU
96% AFUE → 46,000 / 0.96 = 47,917 → 50,000 BTU
Rule-of-thumb estimate. For code-compliant sizing, request a Manual J calculation from a licensed HVAC contractor.
Reference
Common furnace sizes at a glance
Quick scan for the most common scenarios. Sizes shown are input (nameplate) BTU, the rating furnaces are sold by, at the AFUE the calculator recommends for each case. For your home's exact number, use the calculator above.
Small
~1,000 sq ft
40 kBTU
Condo or starter home. 60 kBTU in cold zones.
Average
~1,800 sq ft
60 kBTU
Most US single-family homes. 100 kBTU in cold zones.
Large
~2,500 sq ft+
80–100 kBTU
Larger homes. Step up to 120 kBTU in cold climates.
Learn
Understanding Furnace Sizing
How many BTUs per square foot for heating?
Heating swings far more by climate than cooling does. The colder it gets outside, the harder your furnace works to hold the same indoor temperature. In the mild South you're looking at 18–25 BTU/sq ft. Across the Midwest and Mid-Atlantic it's 30–45, and in the genuinely cold places like Minnesota or northern Maine, 45–60. Treat those as starting points. A drafty house or a vaulted ceiling pushes you toward the high end, which is why the calculator above asks about both.
Output BTU vs. input BTU (and why AFUE matters)
A furnace's output BTU is the heat it actually delivers to your home. Its input BTU is the energy it consumes, always larger because no furnace is 100% efficient. AFUE (Annual Fuel Utilization Efficiency) is the ratio: an 80% AFUE furnace turns 80,000 input BTU into 64,000 output BTU. A 96% AFUE unit needs ~67,000 input BTU to deliver the same output.
Climate zone is the biggest driver
You can size cooling mostly by the house; you have to size heating by the climate. A 1,500 sq ft home in Minneapolis needs nearly two and a half times the heating capacity of the same home in Atlanta, even though the floor plans are identical. The reason: cooling load tracks floor area, while heating load scales with the gap between your indoor temperature and the coldest design day, which varies enormously by region.
Why insulation and ceiling height change the math
Heat escapes through the building envelope. A poorly insulated home loses heat 25–30% faster than a well-insulated one of the same square footage, so it needs a larger furnace. Ceiling height matters because you heat volume, not floor area: vaulted 12 ft ceilings hold 50% more air than standard 8 ft ceilings, and the furnace has to keep all of it warm.
Oversizing costs more than you'd think
An oversized furnace heats the home in short bursts, hits setpoint, and shuts off. Then the temperature drops and it kicks back on. This short-cycling wears out the heat exchanger and igniter faster, wastes fuel on startup losses, and creates uncomfortable hot/cold swings. Right-sized equipment runs longer, gentler cycles and lasts years longer. If you must round, round down.
FAQ
Common Questions About Furnace Sizing
How do I calculate what size furnace I need?
Take your home's square footage and multiply it by a BTU/sq ft rate set by your climate: 18–25 in the mild South, 30 in moderate zones, 45–60 where winters are serious. Then adjust for insulation quality and ceiling height, which is exactly what the calculator above does. A 1,500 sq ft home in a moderate climate works out to about 45,000 BTU output, which a 60 kBTU furnace covers. For a more accurate whole-home figure, use the HVAC Load Calculator.
How many BTUs per square foot for a furnace?
Roughly 18–25 BTU/sq ft in the warm South, 30 BTU/sq ft in moderate climates, and 45–60 BTU/sq ft in cold climates. These are output BTU per sq ft. The number on a furnace nameplate is usually input BTU, so divide by AFUE to compare.
What's the difference between input and output BTU?
Input BTU is the fuel energy a furnace consumes. Output BTU is the heat it actually puts into your house. The gap is efficiency loss out the flue. An 80,000 input BTU / 80% AFUE furnace delivers 64,000 output BTU; an 80,000 input BTU / 96% AFUE furnace delivers 76,800 output BTU. Always size based on output.
Is 80% or 96% AFUE better?
It depends on climate. In cold zones with long heating seasons, a 96% AFUE condensing furnace usually earns back its higher upfront cost through lower fuel bills, and the longer and colder the winters, the sooner that happens. In mild climates, 80% AFUE is usually the better value, because the heating season is too short to recoup the premium. 96% AFUE units also need a special PVC vent and condensate drain, which can complicate retrofits.
Can a furnace be too big for a house?
Yes, and oversized is a more common problem than undersized. Some margin is unavoidable because furnaces come in fixed sizes, but a unit well beyond the smallest size that covers your load will short-cycle, wearing out components and creating uneven temperatures. It also costs more to buy and to run. Going one size up "just in case" almost always backfires.
Related Tools
AC Size Calculator
Pair your furnace with right-sized cooling capacity.
↳ 1,500 sq ft · moderate → ~2.5 ton
HVAC Load Calculator
Whole-home heating + cooling load using simplified Manual J.
↳ 2,000 sq ft · cold → ~90,000 BTU heating
BTU Calculator
Room-level BTU sizing with windows, walls, and occupants.
↳ 15×15 room · 2 windows → ~6,500 BTU