Furnace Size Calculator

Find the furnace BTU sizing that fits your needs with this furnace size calculator.

Cooling needed2.6 tons31,200 BTU/h — buy a 3 ton unit

Nominal unit size3 tons36,000 BTU/h, the next half-ton up

Heating needed75,000 BTU/h21.98 kW · 75 MBH

Furnace size at 80% efficiency93,750 BTU/h input

Boiler size86,250 BTU/hwith 15% for pick-up and distribution

Cooling load per square foot20.8 BTU/h

Square feet per ton577400–600 is typical; much below 400 suggests something is wrong

Conditioned volume12,000 cu ft

Window contribution3,000 BTU/h150 sq ft of glazing at 20 BTU/h each

The formula

BTU/h = area × load factor; 1 ton of cooling = 12,000 BTU/h

Oversizing is worse than undersizing

An air conditioner that is too large cools the air quickly, satisfies the thermostat, and shuts off before it has removed much moisture. The result is a house that is cold and clammy, with the compressor short-cycling — which wastes energy and shortens its life.

These figures are a rule-of-thumb estimate. A real load calculation — Manual J in the US — accounts for the orientation of every wall, the specific windows, the measured air leakage and the local design temperatures, and it routinely comes out 20–30% below what a rule of thumb suggests.

Note the 400–600 square feet per ton range. If your answer falls far outside it, check the inputs before checking the arithmetic.