Furnace Size Calculator: Heating BTU/h and AFUE

The heating model estimates envelope UA from area x zone heating rate x insulation factor x max(1, ceiling height / 8), divided by (70 deg F - zone winter temperature). Conduction is then UA x actual indoor-to-outdoor delta T. Infiltration adds 1.08 x (area x ceiling height x natural ACH / 60) x delta T. The 1.08 constant assumes standard air near sea level. Zone temperatures are illustrative profiles; use local ASHRAE design weather for a full calculation. AFUE is seasonal efficiency; using it to estimate steady rated output is a shortcut. This tool lists 30,000, 40,000, 60,000, 80,000, 100,000, and 120,000 BTU/h input, and returns a candidate only when estimated output is 100-140% of load. Availability varies by manufacturer.

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.

First suitable listed furnace input at a 95% AFUE proxy, average insulation, 8-ft ceilings, 0.5 natural ACH, and 70°F indoor design. These rows use the calculator’s zone profiles and 100–140% screen. No candidate can mean demand is too high or the next step is too oversized. Staging is a part-load feature; it does not add missing maximum capacity. Compare additional rated outputs or separately designed systems.

• 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 — calculate duct leakage and conduction outside the conditioned envelope rather than adding a universal percentage.
• 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 separates estimated envelope loss and natural infiltration. Full Manual J uses measured assemblies and local weather; this is a screening tool.

Furnaces are sold by input rating, while the heating load is required output. Using AFUE as a planning proxy, 70,000 BTU/h output divided by 0.96 gives about 72,917 BTU/h input; at 80%, it gives 87,500. This shortcut changes the fuel-input number, not the building load. Compare manufacturer rated output before choosing the next input step, then check the output-to-load ratio and any altitude derating. The calculator’s short list does not cover every product. A model with another input rating may fit better than the next listed candidate, especially for a small load.

Check maximum and minimum rated output. A single-stage burner operates at its full firing rate on a heat call, so excessive capacity can cause short cycles. Two-stage equipment offers a lower stage, while modulating models track a wider range of demand. Low-stage percentage and turndown are model-specific. Ask how the thermostat calls stages, what airflow each needs, and whether low output suits mild-weather demand. Better turndown can improve comfort after a correct peak-load selection, but it does not remove the applicable output-to-load limit.

Fuel type changes the equipment checks and energy-cost comparison. A gas furnace is selected from rated output with required installation and altitude corrections. Electric resistance heat is sized from electrical output and available circuit capacity. A heat pump needs winter design-temperature output data, defrost considerations, and a backup strategy where required. Local prices and building demand determine costs; do not infer running cost from fuel type alone. Read the heat pump vs furnace comparison and what size furnace guide for the decision process.

A right-sized furnace needs enough airflow to stay within its manufacturer temperature-rise range. Sensible Q = 1.08 × CFM × ΔT near standard air conditions: 80,000 BTU/h output at a 50°F rise needs about 1,481 CFM. This is an illustration, not a universal setting. Check the blower table at actual external static pressure, filter resistance, and coil pressure drop. Seal and insulate ducts outside the envelope and calculate distribution losses instead of adding a fixed percentage. If the blower also serves cooling, verify that airflow independently with the CFM calculator and a duct design.

Confirm combustion air, venting, fuel supply, and condensate provisions before selecting an input rating. Vent category and approved materials depend on the model; a condensing unit may require approved plastic or another listed vent system rather than generic PVC. Follow the installation manual for sizing, equivalent length, termination spacing, and condensate drainage. Combustion air follows applicable fuel-gas code and installation requirements; whole-house ventilation is a separate concern. Also evaluate shared chimneys and other appliances when replacing equipment. A load calculation does not establish that the mechanical room can feed and vent the chosen burner.

Defaults: 2,000 sq ft, 8-ft ceilings, zone 4, average insulation, 0.5 natural ACH, 70 deg F indoors, 95% AFUE. Heating load = 70,000 + 7,488 = 77,488 BTU/h. Estimated input = 77,488 / 0.95 = 81,566 BTU/h after rounding. The next listed input is 100,000; estimated output = 100,000 x 0.95 = 95,000 BTU/h, or 122.6% of load.

Commonly cited Manual S furnace guidance is roughly 100-140% of heating load using output, not input. Confirm manufacturer output and altitude derating before applying that range. If no listed size fits, the tool reports that instead of silently recommending a 120,000-BTU/h furnace that may be too small or oversized.

Inspect conduction and infiltration separately.

ACCA Manual J is the residential load-calculation reference, Manual S covers equipment selection, and Manual D covers ducts. The area rates and adjustment percentages in this tool are site assumptions, not values supplied by those standards.

ACCA Manual S: residential equipment selection. For the load-calculation workflow, read how to calculate HVAC load.

Use the Manual S calculator to compare residential design loads with manufacturer capacity data. Confirm the applicable edition and equipment-specific limits. Nominal labels and this page's screening assumptions are a starting point.