🔥What size furnace does this house need?

✓ Tested & verified Updated: Aug 8, 2026 How we calculate this

Furnace size calculator

Floor area (sq ft)

Ceiling height (ft)

Climate zone

Insulation

100,000 BTU/h≈ 29.3 kW

The room works out at 84,600 BTU/h — 100,000 is the nearest size actually sold.

Floor area: 1,800 sq ft
Working rate: 47 BTU/h per sq ft

The answer is output. A furnace labelled 100,000 BTU at 80% AFUE delivers 80,000 — divide the output you need by the efficiency to get the input rating to shop for.

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What size furnace do you need?

Floor area × the heating rate for your climate zone. An 1,800 sq ft house in zone 5 — Chicago, Boston — at 47 BTU/h per square foot needs about 84,600 BTU/h of heat delivered, so the 100,000 BTU/h output furnace is the size on the ladder above it.

Heating is where the climate zone stops being a detail. The same house wants 27 BTU/h per square foot in Miami and 57 in northern Maine, because the load follows the gap between the temperature you want inside and the coldest the outside gets. That is a factor of two, and no amount of care about ceiling height will make up for using the wrong band.

Then convert output to input. Furnaces are advertised by input: a “100,000 BTU furnace” at 80% AFUE delivers 80,000 BTU/h into the house. Take the output you need and divide by the AFUE — 84,600 ÷ 0.80 is a 105,000 input furnace, or ÷ 0.96 for a condensing model, an 88,000.

Furnace sizing is one table with two axes, and both of them matter. Read down for your climate, across for your house.

Conditioned area	Zone 2	Zone 4	Zone 5	Zone 7
1,000 sq ft	40,000	60,000	60,000	60,000
1,500 sq ft	60,000	80,000	80,000	100,000
2,000 sq ft	80,000	100,000	100,000	120,000
2,500 sq ft	80,000	120,000	120,000	140,000
3,000 sq ft	100,000	140,000	140,000	140,000+

All figures are output BTU per hour, already rounded to a size that is sold. For the model number to shop for, divide by the AFUE: an 80% furnace needs a 25% larger input rating than the output shown, a 96% condensing furnace almost none.

Frequently asked questions

How many BTU per square foot do I need to heat a house?

Between about 27 and 57 BTU per hour per square foot depending on climate zone, for an average-insulation house. Zone 1 on the Gulf coast sits at the bottom, zone 7 in northern Minnesota at the top. A well-sealed modern build sits below its zone’s figure and an uninsulated older one above it, which is why insulation is applied as a separate correction.

What size furnace for a 2,000 square foot house?

In zone 3 that is about 74,000 BTU/h of output and you buy the 80,000. In zone 5, 94,000 and you buy the 100,000. In zone 7, 114,000 and you buy the 120,000. Same house, three answers a size and a half apart — which is exactly why national rules of thumb for furnaces are unreliable.

What is the difference between input and output BTU?

Input is the fuel energy going in; output is the heat reaching the house. AFUE is the ratio. An 80% AFUE furnace with a 100,000 BTU input delivers 80,000 BTU of heat and sends the other fifth up the flue. Model numbers usually quote input, sizing calculations always mean output, and mixing the two by 20% is a very easy mistake to make.

Is a bigger furnace better for very cold snaps?

No. Sizing is done for the design temperature — roughly the coldest 1% of hours in a normal year — and a correctly sized furnace simply runs longer on the rare colder day. An oversized one blasts heat, satisfies the thermostat in minutes, shuts off, and repeats: uneven temperatures, more noise, more wear on the igniter and blower, and no better performance when it matters.

How much oversizing is acceptable?

Twenty to thirty percent above the calculated load is the normal allowance, covering recovery from a setback and a colder-than-design night. That is roughly one step on the size ladder. Two steps, or double, is the level at which comfort measurably gets worse.

Does a two-stage or modulating furnace change the size?

It changes how much oversizing you get away with, not the load. A modulating furnace can run at 40% of capacity, so it behaves well across a wide range of conditions and a slightly generous size costs little. A single-stage furnace only has full output and off, so the calculation deserves more care.

How we calculate this

Heating output starts from a BTU per hour per square foot rate by IECC climate zone — 27 in zone 1, 32 in zone 2, 37 in zone 3, 42 in zone 4, 47 in zone 5, 52 in zone 6 and 57 in zone 7 — applied to conditioned floor area and scaled by ceiling height ÷ 8. Insulation applies the standard +15% / −10% correction. The result rounds up to the output capacities furnaces are built in: 40,000, 60,000, 80,000, 100,000, 120,000 and 140,000 BTU/h. All figures on this page are output; converting to the input rating printed on a model number means dividing by the AFUE. This is a rule of thumb — an ACCA Manual J calculation measures the actual building.

Worked example

With Floor area (sq ft) 1800, Ceiling height (ft) 8, Climate zone Zone 5 — Chicago, Boston, Insulation Average — typical home, this page works out 100,000 BTU/h ≈ 29.3 kW. The room works out at 84,600 BTU/h — 100,000 is the nearest size actually sold.

Floor area1,800 sq ft
Working rate47 BTU/h per sq ft

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