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Furnace Size Calculator

A furnace size calculator estimates the BTU rating a gas furnace needs for your home — both the heat output and the input BTU you'll see on the nameplate — from the area, your climate and the furnace's AFUE efficiency.

Your measurements

Result

Furnaces are sold by input BTU, but what heats the house is the output (input × efficiency). This tool works both out and rounds to a standard size so you can shop confidently.

How it is calculated

Output BTU = area × a climate rate (~30 warm / 40 moderate / 50 cold per sq ft). Input BTU = output ÷ AFUE. We round the input up to a standard furnace size (40k-120k). A Manual J is the precise sizing method; oversizing a furnace causes short-cycling.

Steps

Enter the heated floor area of the home.
Pick your climate zone.
Enter the furnace's AFUE efficiency (90-97% for modern units).
Read the output BTU needed and the input rating to shop for.
Have an installer confirm with a Manual J before buying.

Worked example

An 1,800 sq ft home in a moderate climate needs about 72,000 BTU of output; at 95% AFUE that's ~76,000 input — so an 80,000 BTU furnace.

Common questions

What size furnace do I need?

Roughly 30-50 BTU of output per square foot by climate, then divided by AFUE for the input rating. An average home lands around 60-100k BTU.

What is AFUE?

Annual Fuel Utilization Efficiency — the percentage of fuel turned into usable heat. A 95% AFUE furnace delivers 95% of its input as heat.

Output or input BTU?

Furnaces are advertised by input. Output (what heats the home) = input × AFUE — so a 100k input at 95% delivers 95k.

Is bigger safer?

No — an oversized furnace short-cycles, wastes fuel and wears faster. Size it to the load.