Estimate heat loss through walls, ceiling and windows plus air leakage, using the temperatures you design for.
How it works
For each surface, area divided by R-value times the temperature difference gives BTU per hour. Air leakage adds 0.018 × air changes × house volume × temperature difference.
Before you order
- Find your local design temperature for the outdoor number.
- Drafty older homes often have far higher air change rates than new tight ones.
- Ducts in unheated spaces add losses this calculator does not include.
Common questions
Can I size a furnace from this?
It gives a useful starting point, but equipment should be sized from a Manual J load calculation by a licensed contractor.
Why is the ceiling so much smaller than the walls?
Ceilings usually have far more insulation than walls, so they lose less per square foot.