Furnace Size Calculator

Estimate the furnace BTU size for your home from square footage, climate zone, insulation, ceiling height and AFUE efficiency.

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Results will appear here once you enter your values.

How the estimate is calculated

Required heat output (BTU/h) = heated square feet × climate-zone factor × insulation factor × (ceiling height ÷ 8).

Required furnace input = output ÷ AFUE.

The climate-zone factors are the square-foot rules HVAC contractors use for a first estimate: roughly 30-35 BTU/h per sq ft in zone 1 (South Florida, Hawaii), 45-50 in zone 4 (Washington DC, St.

Louis, Seattle), and 60-65 in zone 7 (northern Minnesota and Maine).

Example: a 2,000 sq ft zone 4 house with average insulation needs about 2,000 × 47.5 = 95,000 BTU/h of heat output, which a 95% AFUE furnace delivers from a 100,000 BTU/h input rating.

Input vs output: reading the nameplate

Gas furnaces are sold by input BTU, the fuel they burn per hour.

What heats the house is output: input × AFUE.

An 80,000 BTU input furnace at 80% AFUE delivers 64,000 BTU/h; the same input at 96% AFUE delivers 76,800 BTU/h.

When comparing quotes, compare output (sometimes listed as heating capacity) against your heat loss, not the headline model number.

What AFUE means

Annual Fuel Utilization Efficiency is the share of a year's fuel that becomes heat in the home, per the U.S.

Department of Energy test procedure.

Non-condensing furnaces are about 80% AFUE, and the rest leaves through the flue.

Condensing furnaces recover heat from water vapor in the exhaust and reach 90-98.5%.

The federal minimum for non-weatherized gas furnaces is currently 80%, with a DOE rule raising it to 95% for furnaces made from late 2028, and ENERGY STAR-certified gas furnaces are 95% AFUE or higher.

Higher AFUE reduces fuel use but does not change the heat output your house needs.

Why square-foot rules often oversize

Rules of thumb assume an average existing house.

A tight, well-insulated home built to a recent energy code can need half the BTUs per square foot of a drafty 1950s house in the same town, and contractors who replace like for like often install furnaces 50-100% larger than the load.

Oversized furnaces short-cycle, create temperature swings, and wear igniters and heat exchangers faster.

ACCA Manual S allows furnace output of 100-140% of the calculated heating load, so use the insulation setting honestly and treat the result as an upper bound for a well-sealed home.

Get a Manual J before you buy

A Manual J heat-loss calculation uses your local 99% winter design temperature, wall, attic and window R-values, window area, and measured or estimated air leakage to size the furnace to the house.

Many utility rebate programs require one.

If you have past gas bills, a contractor can also estimate the load from actual winter fuel use, which captures how the house really performs.

Frequently Asked Questions

Common questions about the Furnace Size Calculator

About 60,000-130,000 BTU/h input depending on climate and insulation. With average insulation in climate zone 4 the heat output needed is about 95,000 BTU/h, or a 100,000 BTU input furnace at 95% AFUE. A well-insulated home in zone 3 might need only 60,000.

Sources & References

Furnaces and Boilers

AFUE definition, efficiency ranges for non-condensing and condensing furnaces, and sizing guidance.
→Source: U.S. Department of Energy, Energy Saver

Manual J: Residential Load Calculation

Standard method for calculating a home's heating load.
→Source: ACCA

Manual S: Residential Equipment Selection

Rules for matching furnace output to the Manual J load (100-140% for furnaces).
→Source: ACCA

IECC Climate Zone Map

County-level U.S. climate zones used to pick the BTU per square foot factor.
→Source: Pacific Northwest National Laboratory, Building America Solution Center

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