Furnace Size Calculator

Sizing a furnace is a two-step process. First you find the home's design heating load — the BTU/hr of output needed to hold the indoor temperature on the coldest design day. This uses the same conduction-plus-infiltration method as our heat load calculator. Then you convert that output into the furnace's input rating by dividing by AFUE, because some fuel energy is always lost up the flue.

Picking a furnace is the heating side of whole-system design. If you also need cooling capacity, the heat pump vs furnace comparison explains when an electric heat pump replaces or supplements a gas furnace, and our what size furnace guide walks through a full worked example.

Nearest standard furnace input at 95% AFUE, average insulation, 8-foot ceilings, 0.5 ACH, 70°F indoor design. Leaky homes (1.0 ACH) often jump one size; excellent insulation can drop one. Homes above 120,000 BTU/hr usually move to a two-stage or modulating unit, or a second furnace.

• Confusing input and output — buying an 80,000 BTU furnace when your load is 80,000 BTU of output, which at 95% AFUE actually needs ~84,000 input.
• Sizing by square footage alone — the same house needs very different capacity in zone 3 versus zone 7.
• Matching the old furnace's size — original equipment was often oversized, and your insulation or windows may have since improved.
• Ignoring duct losses — ducts in an unconditioned attic or crawlspace can add 15–25% to the required output.
• Oversizing 'to be safe' — a furnace double the load short-cycles, wears parts, and feels drafty between cycles.
• Using the wrong design temperature — sizing to the record low instead of the 99% design temperature wastes capacity you'll almost never use.
• Forgetting AFUE when comparing quotes — an 80% and a 96% furnace with the same input rating do not deliver the same heat.

The single idea that separates a correctly sized furnace from a guessed one is this: capacity follows the heating load, never the square footage. The load is the rate at which a house loses heat on a cold day, and it is set by the envelope — wall and ceiling R-values, the U-value of the windows, and how much outdoor air leaks in — working against the temperature difference between your thermostat setting and the outdoor design temperature. Two 2,000 sq ft homes can differ by a factor of two or three in load depending on insulation, glazing, and climate. The formal way to settle it is an ACCA Manual J room-by-room load calculation, which is the residential method referenced by the IECC and most building codes. Equipment selection then follows ACCA Manual S, which says to choose a unit whose output meets the design load without grossly exceeding it.

The outdoor side of that math is the 99% heating design temperature — the temperature your area stays above 99% of the hours in a year, taken from the ASHRAE Handbook of Fundamentals climate tables rather than the record low. Sizing to a once-a-decade extreme buys capacity you will almost never use and worsens the day-to-day comfort penalty of an oversized burner. You can estimate the underlying load with our heat load calculator, which uses the same conduction-plus-infiltration approach a Manual J tool applies at the design temperature for your IECC climate zone.

Manufacturers and AHRI list furnaces by input BTU/hr, so once you know the delivered output your home needs, divide by AFUE to find the rating to buy. A 70,000 BTU/hr output load met by a 96% AFUE condensing furnace needs about 73,000 BTU/hr input; the same load met by an 80% AFUE non-condensing unit needs 87,500. AFUE — the DOE and ENERGY STAR efficiency metric — therefore changes the input number you shop for, not the load itself. Residential gas furnaces come in fixed input steps, typically 40,000, 60,000, 80,000, 100,000, and 120,000 BTU/hr, and you take the smallest step at or above your requirement.

How the furnace modulates its firing rate determines how comfortably it lives with that fixed step. A single-stage furnace fires at 100% every time the thermostat calls, which makes any oversizing land hard as short cycling. A two-stage furnace runs a low stage (often around 65–70% of input) for most of the season and only steps up on the coldest days. A modulating furnace can ride anywhere from roughly 35% to 100%, holding a near-constant supply temperature and the longest, quietest run times. The more a unit can turn down, the more forgiving it is of landing one standard size above your calculated load.

Fuel type frames the whole choice. A gas furnace delivers its full rated output at any outdoor temperature, so you size it straight to the design load. An electric furnace converts resistance heat at effectively 100% efficiency but at electricity prices that are usually higher per delivered BTU, and a heat pump loses capacity as it gets colder, so it is sized against a low-temperature performance table instead. Our heat pump vs furnace comparison walks through that trade-off, and the what size furnace guide carries a full worked example from load to input rating.

A right-sized furnace still underperforms if the duct system cannot carry its airflow. Furnaces are blower-rated for roughly 100–130 CFM of air per 10,000 BTU/hr of output, and undersized or leaky ducts raise static pressure, cut airflow, and can trip the high-limit switch. Duct sizing has its own standard — ACCA Manual D — and where supply trunks run through an unconditioned attic or crawlspace, sealing and insulating them matters because distribution losses can add 15–25% to the heat you actually need to produce. The blower you inherit with a given furnace size also sets what your future cooling coil can move, so it is worth checking against the airflow that system will demand.

Combustion and venting are the other practical limits. A non-condensing 80% AFUE furnace vents hot flue gas through a metal Category I chimney or liner, while a 90%+ condensing unit vents cooler exhaust through PVC and drains acidic condensate, which needs a trap and often a neutralizer and a floor drain. Either type needs adequate combustion air; a furnace starved of air in a tight mechanical closet can backdraft, and ASHRAE 62.2 whole-house ventilation guidance and local fuel-gas codes both bear on how that make-up air is provided. Confirm that gas line capacity, the flue path, and combustion-air openings all suit the input rating before you commit to a size — the cleanest load calculation is wasted if the installation cannot feed and vent the burner you chose.