FreeHVACCalculators

Furnace Sizing Calculator

Estimate how many BTUs per hour a house loses on a design-cold day, and what furnace input rating delivers that at your chosen efficiency.

Method reviewed October 22, 2026. Version 1.0.0. How it works

Your inputs

Unit system
Stories above grade

Representative design conditions for each IECC climate zone. Choose Custom to enter your local values.

12–18 % is common for houses.

Condensing furnaces are typically 90–98 % AFUE; older non-condensing furnaces about 78–83 %.

Result

Design heat loss (required output)
42,487BTU/h
Minimum furnace input rating
44,723BTU/h

The house loses about 42,487 BTU/h at design conditions. A 60,000 BTU/h input furnace at 95 % AFUE delivers about 57,000 BTU/h.

Suggested standard input size
60,000BTU/h
Output of suggested size
57,000BTU/h
Output ÷ heat loss
1.34

Heat loss breakdown

Walls
5,555BTU/h
Ceiling / roof
3,733BTU/h
Windows (conduction)
8,232BTU/h
Doors
896BTU/h
Slab edge
7,313BTU/h
Air leakage (infiltration)
9,677BTU/h
Duct losses / gains
7,081BTU/h
Show the working
  1. Footprint and perimeter (square plan)
    2,000 ft² / 1 = 2,000 ft²; perimeter = 4 × √2,000 = 178.9 ft
  2. Net wall area
    1,431 ft² gross − 300 ft² windows − 40 ft² doors = 1,091 ft²
  3. Conditioned volume
    2,000 ft² × 8 ft = 16,000 ft³
  4. Heating design temperature difference
    70 °F − 14 °F = 56 °F
  5. Design heat loss including ducts
    42,487 BTU/h
  6. Required input = heat loss ÷ AFUE
    42,487 / 0.95 = 44,723 BTU/h

Heat loss first, furnace second

A furnace has two ratings: input (the fuel it burns) and output (the heat it delivers). Output = input × AFUE. The house’s heat loss at the design temperature sets the output you need; dividing by AFUE gives the input rating that appears in model numbers.

For example, a 60,000 BTU/h-input furnace at 96 % AFUE delivers about 57,600 BTU/h, while the same input at 80 % AFUE delivers only 48,000 BTU/h.

What goes into the heat loss

Heat leaves through walls, ceiling, windows and doors by conduction (U × A × ΔT), through outdoor air leaking in, and through the floor — along the edge of a slab, or into a crawlspace. Ducts in unheated spaces add to the loss. The calculator totals these at your climate’s 99 % heating design temperature and an indoor temperature of 70 °F.

Picking a size

We suggest the smallest standard input size whose output covers the heat loss. If even that size is far larger than the loss — common in small, well-insulated homes — a modulating or two-stage furnace, or a heat pump, will run longer, quieter and more evenly.

In cooling-dominated homes the furnace is often chosen for the blower airflow the air conditioner needs rather than for heat loss. Check both.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Conditioned floor area
2,000 ft²
Stories above grade
1 story
Average ceiling height
8 ft
Climate
Zone 4 — mixed (e.g. Baltimore)
Insulation & air sealing
Average (code-built 1990s–2000s)
Window type
Double pane, clear
Window area (% of floor area)
15 %
Foundation
Slab on grade
Duct location
Attic/crawl — typical
Occupants
4
Furnace efficiency (AFUE)
95 %

Result

Design heat loss (required output)
42,487 BTU/h
Minimum furnace input rating
44,723 BTU/h
Suggested standard input size
60,000 BTU/h
Output of suggested size
57,000 BTU/h
Output ÷ heat loss
1.34

The house loses about 42,487 BTU/h at design conditions. A 60,000 BTU/h input furnace at 95 % AFUE delivers about 57,000 BTU/h.

Assumptions

  • Indoor design conditions are 70 °F for heating and 75 °F for cooling (ACCA Manual J defaults).
  • The house is modelled as a square footprint with two 20 ft² doors; homes with complex shapes have more wall area and a somewhat higher load.
  • Opaque surfaces use an equivalent temperature difference for sun-heated surfaces: walls +10 °F, ceilings under a vented attic +30 °F.
  • Window solar gain uses an all-orientation average of 60 BTU/h·ft² (calibrated so typical results fall in the range published Manual J studies report) multiplied by the window SHGC and an interior-shading factor of 0.7.
  • Summer air leakage is taken as half of the winter natural air-change rate.
  • Each occupant adds 230 BTU/h sensible and 200 BTU/h latent heat; appliances and lighting add 1,200 BTU/h.
  • Duct losses are a fixed percentage of the sensible load depending on duct location (0 %, 10 %, 20 % or 30 %).
  • Furnace output = input × AFUE. AFUE is a seasonal rating, so steady-state output can differ slightly.

Limitations

  • This is a screening estimate, not an ACCA Manual J load calculation. Use it to sanity-check a quote or plan a project, not to finalise equipment selection.
  • Orientation, shading by trees or neighbouring buildings, actual window placement, and room-by-room loads are not modelled; a whole-house figure cannot size individual duct runs.
  • Climate presets are rounded values for one representative city per zone. Design temperatures vary considerably within a zone — enter your local values for a better estimate.
  • Conditioned basements are not modelled; below-grade wall losses are excluded.
  • Furnace sizing for cooling-dominated homes is often set by the blower airflow the air conditioner needs, not by heat loss.

Frequently asked questions

How many BTUs per square foot for a furnace?

Rules of thumb range from about 30 to 60 BTU/h per square foot by climate, but insulation, windows and air leakage move the real figure by a factor of two or more. That is why this calculator estimates heat loss from those factors instead.

Is an oversized furnace a problem?

Yes. It short-cycles, which wastes fuel during warm-up, makes temperatures swing, and stresses the heat exchanger. It can also be paired with ductwork too small for its airflow, which pushes the temperature rise above the rated range.

What AFUE should I enter?

The AFUE on the furnace’s rating plate or spec sheet. New condensing gas furnaces are typically 90–98 % AFUE. Federal minimums are 80 % for non-weatherized gas furnaces today, rising to 95 % for units made from December 2028.

Sources

  1. Manual J — Residential Load Calculation — Air Conditioning Contractors of America (ACCA). Manual J is the ANSI-recognised standard for residential equipment-sizing loads required by building codes.
  2. Proper Sizing of HVAC Systems — U.S. DOE Building Science Education Solution Center. Rules of thumb lead to oversized systems; loads should be calculated with Manual J.
  3. Heat Loss from Buildings — The Engineering ToolBox. Conduction heat loss Q = U·A·ΔT with U = 1/ΣR.
  4. Calculating heat loss by infiltration (factsheet, 1988) — Washington Energy Extension Service (via AIVC). Infiltration heat loss Q = 0.018 × ACH × volume × ΔT.
  5. 10 CFR 430.32 — Energy and water conservation standards — U.S. Code of Federal Regulations (Cornell LII). Federal minimum furnace AFUE (80 % non-weatherized gas; 95 % for units made from 18 Dec 2028).
  6. What is AFUE? — Carrier. AFUE is the share of fuel energy delivered as heat over a season.

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