How we worked it out
Updated as you change the numbers.
- BTU/h = area × BTU per sq ft for the climate × (ceiling height ÷ 8 ft) × insulation factor.
Method and assumptions
Rules of thumb of roughly 30 to 60 BTU per hour per square foot, by climate, are widely used for rough sizing. They ignore window area, orientation, air leakage and duct losses, so a Manual J calculation is the right way to choose a furnace or heat pump.
Questions
What size furnace for 1,800 sq ft?
Roughly 81,000 BTU per hour in a cool climate (Zone 5) by this rule; a Manual J load calculation is often lower for a well-insulated house.
Sources
Method and figures checked on . Every worked answer on this page is covered by an automated test. Found a mistake? Tell us.