A house loses heat in two ways: through its surfaces, in proportion to their area, their U-factor and the temperature difference, and through air leaks, as cold air replaces warm. Adding the two at the design temperature tells you how big a furnace or heat pump needs to be.
How to use this calculator
- Enter the heated floor area and the ceiling height.
- Enter the outside wall area and the window and door area within it, and the R-values; the R-value calculator gives the wall's.
- Choose how tight the house is and the outdoor design temperature for your area.
Worked examples
A typical house in a cold climate
Heated floor area: 1,800 sq ft · Outside wall area, windows included: 1,500 sq ft · Wall R-value: 13 · Window and door area: 250 sq ft · Windows: Double pane, clear · Ceiling or attic R-value: 38 · Air tightness: Average · Outdoor design temperature: 0 °F
- Heat loss
- 27,221 BTU per hour
- Per square foot of floor
- 15.1 BTU per hour
- In kilowatts
- 8.0 kW
An older, drafty house
Heated floor area: 1,500 sq ft · Outside wall area, windows included: 1,300 sq ft · Wall R-value: 6 · Window and door area: 220 sq ft · Windows: Single pane · Ceiling or attic R-value: 19 · Air tightness: Loose (older, drafty) · Outdoor design temperature: 10 °F
- Heat loss
- 39,270 BTU per hour
- Per square foot of floor
- 26.2 BTU per hour
- In kilowatts
- 11.5 kW
A new, tight house
Heated floor area: 2,200 sq ft · Outside wall area, windows included: 1,700 sq ft · Wall R-value: 20 · Window and door area: 300 sq ft · Windows: Double pane, low-e · Ceiling or attic R-value: 49 · Air tightness: Tight (new, well sealed) · Outdoor design temperature: -10 °F
- Heat loss
- 19,840 BTU per hour
- Per square foot of floor
- 9.0 BTU per hour
- In kilowatts
- 5.8 kW
A mild climate
Heated floor area: 1,600 sq ft · Outside wall area, windows included: 1,400 sq ft · Wall R-value: 13 · Window and door area: 240 sq ft · Windows: Double pane, clear · Ceiling or attic R-value: 30 · Air tightness: Average · Outdoor design temperature: 30 °F
- Heat loss
- 14,796 BTU per hour
- Per square foot of floor
- 9.2 BTU per hour
- In kilowatts
- 4.3 kW
How heat loss is worked out
Through each surface, the heat loss in BTU per hour is its area times its U-factor times the temperature difference, where U is one over the R-value; Oklahoma State Extension walks through the same method. Windows use their own U-factor: about 0.52 for clear double-pane and 0.28 for low-e.
Air leaks add 0.018 BTU per cubic foot per degree, times the house volume and the air changes per hour. For heating design, a tight house sees about 0.1 to 0.21 air changes an hour, an average one 0.28 to 0.61 and a loose one 0.58 to 1.29.
Picking the design temperatures
Heating is sized for a cold day, not the coldest ever: the design temperature is the one your area stays above 99 percent of the hours of the year. Your local building department or HVAC contractor can give the figure. Indoors, Manual J uses 70 °F.
Common questions
How do I calculate heat loss in my house?
For each wall, window, ceiling and floor, multiply its area by its U-factor (one over its R-value) and by the temperature difference, then add the air leakage. The calculator does all of it.
How many BTUs do I need to heat a house?
As many as it loses on a design day: the heat loss in BTU per hour. It depends far more on insulation, windows and air tightness than on floor area alone.
What is the formula for heat loss?
BTU per hour = area × U-factor × temperature difference for each surface, plus 0.018 × volume × air changes per hour × temperature difference for air leaks.
Is this the same as a Manual J?
No. Manual J goes room by room and accounts for orientation, ducts and more; this is a whole-house estimate to understand where the heat goes and roughly how much heating you need.