Furnace Size Calculator (ACCA Manual S)

Background.

Two quotes arrive for the same house, both for an "80,000 BTU" gas furnace. One is a non-condensing model at 80% AFUE, the other a condensing model at 95%. They are not the same size. Furnaces are sold by input — the rate at which they burn gas — but a house is heated by output, and output is input times efficiency: 64,000 BTU/h from the first, 76,000 from the second. If the house loses 56,000 BTU/h on a design winter day, the first delivers 114.3% of that load and the second 135.7%.

Whether either is acceptable is a question the building code answers by reference. Section M1401.3 of the 2021 International Residential Code requires heating equipment to be sized under ACCA Manual S, using a load calculated under ACCA Manual J or another approved method. Manual S, Section 2-2, sets the window for furnaces and boilers: an output capacity of 100% to 140% of the total heating load. Below 100%, the furnace runs flat out on the coldest days and still loses ground. Above 140%, it satisfies the thermostat in short bursts, with wider temperature swings, rooms far from the air handler that never quite warm, and extra wear on the igniter, inducer and heat exchanger.

This calculator applies that window. Give it a design heating load and it returns the output band, the input rating that corresponds to the bottom of the band at your AFUE, and a verdict on a specific furnace you are weighing up. It also works out the blower airflow that furnace needs at your chosen temperature rise, the figure that decides whether the existing ducts can carry it.

The answer is no better than the load you feed it. A Manual J calculation models walls, windows, insulation, air leakage, orientation and the local winter design temperature. The alternative offered here, floor area times a heating factor, is a rough screening estimate with no code standing: useful for a ballpark while shopping, not acceptable for a permit. Forced-air furnaces only; boilers are rated differently and have their own calculator.

What is furnace size calculator?

Furnace size is a heat-delivery rate in BTU per hour, quoted two ways. The input rating is how fast the burner consumes fuel; the output rating, listed as heating capacity on the AHRI-certified nameplate, is how fast heat reaches the ductwork. Annual fuel utilization efficiency (AFUE), defined in federal regulation at 10 CFR 430.2 and measured under the Department of Energy's appendix N test procedure, links the two as a seasonal percentage. The design heating load is the rate at which the house loses heat at the local winter design temperature — a cold condition the area reaches in a normal winter, not the record low — and it is the number the output must cover. Sizing a furnace means choosing an output inside the Manual S band around that load, then checking that the blower can move enough air within the nameplate temperature-rise range.

How to use this calculator.

1. 01Choose the load source. If you have a Manual J report, an energy audit or a room-by-room heat-loss calculation, pick the first option and enter the total design heating load in BTU/h — not your gas usage and not the rating of the furnace you have now.
2. 02Without a load calculation, pick the floor-area estimate, enter conditioned floor area only (a conditioned basement counts; an unheated garage or vented attic does not) and set a heating factor you can justify. Factors run from about 15–20 BTU/h per sq ft for a tight new house in a mild climate to more than 60 for a leaky older house in a severe one.
3. 03Enter the AFUE from the yellow EnergyGuide label or the AHRI directory: roughly 80–83% for non-condensing furnaces and 90–98% for condensing ones.
4. 04Enter the output (heating capacity) of the furnace you are considering, not the input figure in its model name. If you are still shopping, start with the middle of the recommended band.
5. 05Set the design temperature rise to a value inside the range printed on that furnace's nameplate, usually near the middle; 35–65 °F is a common range.
6. 06Read the band and the verdict, compare the blower airflow with what your ducts can carry, and have a licensed mechanical contractor confirm the load, the selection, the venting, the combustion air and the gas piping.

The formula.

Four relationships produce every number. The band is Q_min = L and Q_max = 1.40 × L, where L is the design heating load in BTU/h; both edges count as inside, because Manual S §2-2 states the range as 100% to 140% inclusive. The verdict compares the candidate's output with the load as a ratio, Q_cand ÷ L, also shown as a percentage, and is decided on the exact ratio rather than the rounded display.

The input figure uses the definition of efficiency: output = input × efficiency, so input = Q_min ÷ AFUE. AFUE is seasonal and already includes cycling and standby losses, so it runs a few points below the steady-state efficiency at which nameplate output is measured. Dividing by it gives an input slightly higher than a real furnace of that output would carry — a small, conservative error, and the reason to size on certified output whenever you have it.

Airflow comes from the sensible-heat equation for air, Q = 1.08 × cfm × ΔT, rearranged to cfm = Q_cand ÷ (1.08 × ΔT). The 1.08 is derived, not fitted: the density of standard air, 0.075 lb/ft³, times its specific heat, 0.24 BTU per lb per °F, times 60 minutes per hour. Manual S takes the heating design airflow from the nameplate temperature-rise range this way, and Manual D then sizes ducts to carry it. Airflow and rise are inversely linked, so halving the rise doubles the airflow required. At high altitude thinner air carries less heat per cubic foot, and the standard 1.08 overstates what each cfm delivers.

In floor-area mode, L is simply area × heating factor. Nothing else in the chain changes, which is why a poorly chosen factor flows straight through to every output.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Wes Davis, Air Conditioning Contractors of America, "Reviewing HVAC Designs for Compliance with ACCA Manual S", ICC Building Safety Journal, January–February 2009, pp. 21–24. Read directly from the PDF on 2026-07-29. Source of the Manual S Table 1 sizing limits used here — furnaces and boilers at 100–140% of total heating load (Manual S §2-2), air conditioners at 115% and heat pumps at 115% or 125% of total cooling load — and of the 56,000 / 78,400 / 64,000 BTU/h worked example this calculator reproduces. Written by ACCA's own technical staff about ACCA's own standard.
2. [2]International Code Council, 2021 International Residential Code, Section M1401.3 "Equipment and appliance sizing": heating and cooling equipment and appliances shall be sized in accordance with ACCA Manual S or other approved sizing methodologies based on building loads calculated in accordance with ACCA Manual J or other approved heating and cooling calculation methodologies. Retrieved 2026-07-29. The section's two exceptions — multistage or variable-refrigerant-flow equipment whose published capacity range brackets the load, and the next-larger-standard-size case — are stated beside the verdict on this page. Adopted editions and local amendments vary by jurisdiction.
3. [3]U.S. Department of Energy, 10 CFR 430.2, definition of "Annual fuel utilization efficiency" — the efficiency descriptor for furnaces and boilers, determined using the prescribed test procedures. The procedure itself is 10 CFR part 430 subpart B appendix N, "Uniform Test Method for Measuring the Energy Consumption of Consumer Furnaces Other Than Boilers", which defines AFUE by reference to ASHRAE 103-1993 §§11.2.12, 11.3.12, 11.4.12 and 11.5.12. Retrieved 2026-07-29. This is the basis for treating AFUE as a seasonal rather than steady-state efficiency on this page.
4. [4]Air Conditioning Contractors of America, ANSI/ACCA 3 Manual S, "Residential Equipment Selection", Section 2-2 (heating equipment sizing limits) and Section 2-6 (heating design airflow from the nameplate temperature-rise range). The standard itself is sold by ACCA and was not opened directly; §2-2's content is taken from the ACCA-authored ICC article cited above and independently confirmed against a RESNET training deck. Listed here as the governing document.
5. [5]ASHRAE Handbook — Fundamentals, chapter on climatic design information and the psychrometric properties of standard air. Source of the standard-air density of 0.075 lb/ft³ and specific heat of 0.24 BTU/lb·°F from which the sensible-heat constant 1.08 = 0.075 × 0.24 × 60 is derived. ASHRAE Handbook access is gated behind purchase or membership; the constant was reconstructed from its published components and checked arithmetically rather than read from the Handbook itself.
6. [6]National Institute of Standards and Technology, "Guide for the Use of the International System of Units (SI)", NIST Special Publication 811, Appendix B.8. Retrieved 2026-07-29. Source of the unit definitions underlying every figure on this page: one British thermal unit (International Table) is 1.055 056 × 10³ J, and one Btu(IT) per hour is 2.930 711 × 10⁻¹ W.