Construction

Furnace Size Calculator

Estimate furnace heating capacity from area.

Calculated Output

Heating needed (BTU/h) 63,000

Tons (for heat pump) 5.25

Formula: BTU = area × BTU/ft²

Step-by-Step Breakdown

1. Values used:

2. Floor area = 1,800

3. Heating load = 35 BTU/ft²

5. Heating needed = Floor area x Heating load = 1,800 x 35 = 63,000BTU/h

6. Tons (for heat pump) = 5.25

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The math behind it

BTU/h = area × ~30–40 BTU/ft² (colder climates need more).

1,800 ft² at 35 BTU/ft² → 63,000 BTU/h.

Manual J?

For a proper size, a contractor does a Manual J heat-loss calculation.