Furnace Size Calculator (BTU/h from last winter’s fuel use and climate)

Understanding the furnace size calculator — concept and principle

This calculator estimates the right furnace size for a house from last year's heating fuel use and heating degree days, instead of guessing from floor area. It gives the design heat load and the range of furnace output and input ratings that match it. Homeowners use it to question an oversized quote before replacing a furnace.

Size from last winter's gas bill: with the default inputs the result is 26,000 BTU/h (design load at 5 °F ≈ 7.6 kW).

How it is calculated

Heat delivered last year = fuel × heat content × old efficiency (AFUE). Heat content (US EIA): natural gas 100,000 BTU per therm, propane 91,452 BTU per gallon, heating oil 138,500 BTU per gallon.
Heat-loss rate UA (BTU/h per °F) = heat delivered ÷ (24 × heating degree days).
Design heat load (BTU/h) = UA × (indoor temperature − outdoor design temperature).
Furnace output: 100% to 140% of the design load (ACCA Manual S).
Input rating = output ÷ new AFUE; kW = BTU/h ÷ 3,412.14.

The principle

A house loses heat roughly in proportion to the temperature difference between inside and outside. Heating degree days add up that difference over a whole season, so dividing the heat your furnace actually delivered by 24 × degree days gives the house's heat-loss rate per degree. Multiplying by the difference on a design-cold day gives the heat the furnace must supply then. ACCA Manual S sizes furnaces at 100–140% of the design load: big enough for the coldest days without short cycling the rest of the year.

Worked example

Default inputs: natural gas, 600 therms, old AFUE 80%, 5,000 °F·days, indoor 70 °F, design outdoor 5 °F, new AFUE 95%.

Heat delivered = 600 × 100,000 × 0.80 = 48,000,000 BTU.
UA = 48,000,000 ÷ (24 × 5,000) = 400 BTU/h·°F.
Design load = 400 × (70 − 5) = 26,000 BTU/h = 7.6 kW.
Output range = 26,000 to 26,000 × 1.4 = 36,400 BTU/h.
Input range = 26,000 ÷ 0.95 = 27,368 to 36,400 ÷ 0.95 = 38,316 BTU/h.

Each point is the calculator re-run with only “Heating degree days last year (°F·day, base 65 °F)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Heating degree days last year (°F·day, base 65 °F)	Design heat load (BTU/h)
1,250	104,000
2,500	52,000
3,750	34,667
5,000	26,000
6,250	20,800
7,500	17,333
10,000	13,000

How to read the result

Compare the output range with the furnace models you are offered. A furnace much larger than 140% of the load runs in short bursts, which can be less comfortable and wear parts faster. If you plan insulation or window upgrades, the load will fall, so size for the improved house. Remember that the nameplate's large number is often the input; check the output.

Common mistakes

Including gas used for hot water or cooking in the heating figure.
Using degree days from a different year or base temperature.
Comparing the input rating with the output range.
Sizing from the old furnace's rating, which may itself be oversized.

Keep in mind: This is a screening estimate. A room-by-room ACCA Manual J calculation by a qualified HVAC professional is needed for the final choice, and installation must follow local codes.

Frequently asked questions

How big a furnace do I need?

One whose output is 100–140% of your design heat load; in the example, 26,000 to 36,400 BTU/h.

What is the difference between input and output BTU?

Input is the fuel energy burned; output is the heat delivered, input × AFUE.

Is a bigger furnace better?

No. An oversized furnace short-cycles and can be less comfortable and efficient.

Sources: US EIA, Energy Explained, "British thermal units (Btu)" · RESNET, "Basics of Manual S" (2014), furnace capacity 100–140% of the Manual J load

Checked 9 Nov 2026. US EIA, “British thermal units (Btu)” in Energy Explained (natural gas 100,000 Btu per therm, propane 91,452 Btu per gallon, heating oil 138,500 Btu per gallon); degree-day relation as in the Heat Loss Calculator (ISO 13789:2017, CIBSE TM41); RESNET, “Basics of Manual S” (2014): furnace total capacity 100–140% of the Manual J load.

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