Furnace Size Calculator
Converts a heat loss figure into the furnace size to buy. Furnaces are sold by input rating, so efficiency has to be taken out before you can compare the number on the label to the heat your house needs.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
input rating = heat loss × (1 + margin) ÷ AFUE
AFUE is the share of the fuel that ends up as heat in the house rather than in the flue, so dividing by it converts the heat you need into the fuel you have to burn. The margin covers reheating a house that has been allowed to cool overnight, which the design load alone does not account for.
What to watch out for
Two numbers on a furnace look similar and are not:
- Input rating is the fuel it burns — the headline figure in the model name.
- Output is the heat that reaches the house, which is the input multiplied by AFUE.
An 80,000 BTU furnace at 80% AFUE delivers 64,000 BTU/h. Sizing against the input rating is the most common way people end up 20–25% oversized without realising it.
Frequently Asked Questions
What size furnace do I need for a 2,000 sq ft house?
In a cool climate with average insulation that is roughly a 70,000 BTU/h heat loss, which at 80% AFUE and a 15% margin means about a 100,000 BTU/h input furnace. A well-insulated house in a mild climate might need half that, so run your own numbers rather than using floor area alone.
What is AFUE?
Annual Fuel Utilisation Efficiency — the proportion of fuel energy that becomes useful heat across a whole heating season. An 80% AFUE furnace sends a fifth of the fuel it burns up the flue; a condensing furnace at 95% sends a twentieth.
How much should I oversize a furnace?
10–25% above the design heat loss. That covers recovery after a night setback. Beyond about 40% the furnace starts short-cycling: it satisfies the thermostat faster than the ducts can distribute the heat, so rooms stay uneven and the burner wears.