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Furnace Size Calculator

Furnace BTU from heated area, climate and efficiency rating.

Your measurementsEvery assumption editable01

Colder climates and poorer insulation both push this up.

Modern condensing furnaces run 90–98%. Older units 60–80%.

Furnace sizeCalculated live02

Result

75789BTU/hr

75789 BTU input — 72000 BTU delivered

Heat output needed1800 sq ft × 40 BTU/sq ft
72000 BTU/hr
Efficiency
95% AFUE
Input rating requiredfurnaces are advertised by input, not output
75789 BTU/hr
Nearest nominal size
60000 BTU/hr

Furnaces are advertised by their input rating while what heats the house is the output. At 95% AFUE the difference is small; at 70% a nominal 100,000 BTU furnace delivers only 70,000, which is why comparing old and new units by their badge numbers is misleading.

Oversizing a furnace causes short cycling and uneven temperatures, though it is less punishing than oversizing air conditioning because heating does not depend on run time for dehumidification.

Fuel type, venting, combustion air and gas line sizing all follow from this figure and none of them are calculated here. A furnace that cannot draw enough gas or combustion air will not deliver its rating.

Enter the heated floor area, a heating load for your climate and the efficiency of the furnace you are considering. The result gives both figures that matter: the heat you need delivered, and the input rating furnaces are actually advertised by.

Why use this tool?What it does differently03

Why use this tool?

Input and output both

Furnaces are sold by input rating but heat a house by output. At 70% AFUE the difference is nearly a third.

Climate heating load

From 25 BTU per square foot in mild regions to 60 where winters are severe.

Efficiency aware

Change the AFUE and the input rating moves — which is how you compare an old furnace against a modern condensing one honestly.

Nearest nominal size

Matched to the sizes furnaces are actually manufactured in.

How this worksThe method04

How this furnace size calculator works

Heated area multiplied by a heating load in BTU per square foot gives the heat that has to reach the rooms. That figure is the output, and it is what determines comfort.

Dividing output by the AFUE efficiency gives the input rating — the number on the furnace badge. At 95% AFUE the two are close; at 70% a nominal 100,000 BTU furnace delivers only 70,000, which is why comparing an old and new unit by their badges alone is misleading.

Oversizing a furnace causes short cycling and uneven temperatures, though it is less punishing than oversizing air conditioning because heating does not depend on run time for dehumidification.

How to use itStep by step05

How to use it

  1. Step 1: Measure heated area

    Floor area the furnace serves, excluding unheated spaces.

  2. Step 2: Pick the heating load

    25 for mild winters, 35 moderate, 45 cold, 60 for severe. Insulation quality shifts this too.

  3. Step 3: Enter the AFUE

    Modern condensing furnaces are 90 to 98 per cent. Older units run 60 to 80.

  4. Step 4: Shop by input rating

    The input figure is what appears on the badge and in the catalogue.

Example usageWorked figures06

Example usage

1,800 sq ft in a cold climate
At 45 BTU per square foot that is 81,000 BTU output. At 95% AFUE the input rating needed is 85,263 — an 80,000 or 90,000 BTU furnace depending on how the rest of the house performs.
Comparing old against new
The same 81,000 output from a 70% AFUE furnace needs 115,714 BTU input. The old unit's larger badge number does not mean it heated better — it means it wasted more.
A mild climate
1,800 sq ft at 25 BTU per square foot needs 45,000 output, or 47,368 input at 95%. Roughly half the severe-climate figure for an identical house.
Frequently asked questionsCommon questions07

Frequently asked questions

What size furnace do I need for 2,000 square feet?

Between 50,000 BTU output in a mild climate and 120,000 in a severe one. The spread is enormous, which is why climate is an input rather than an assumption.

What is the difference between input and output?

Input is the fuel energy the furnace consumes; output is the heat it delivers. AFUE is the ratio. Furnaces are advertised by input, so a 100,000 BTU furnace at 80% AFUE delivers 80,000.

Is a higher AFUE always worth it?

It depends on climate and fuel price. In a cold climate with a long heating season the payback on a condensing furnace is quick; in a mild one it can outlast the equipment.

Can a furnace be too big?

Yes. Oversized furnaces short-cycle, producing uneven temperatures and more wear. It is less damaging than oversized cooling, but it is still a real cost.

Does this include venting and gas sizing?

No. Venting, combustion air and gas line sizing all follow from the input rating and are separate calculations. A furnace that cannot draw enough gas or air will not deliver its rating.

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