Window Heat Loss Calculator
Windows are often the weakest point in a home's thermal envelope. The rate of heat loss through a window depends on its area, its U-factor (how readily it conducts heat), and the temperature difference between inside and outside. This calculator uses the standard conductive heat-loss equation to find the instantaneous heat loss in BTU per hour, and estimates a heating-season energy cost from your fuel price.
Window heat loss formula
Area = (width * height) / 144 (sq ft)
Delta-T = indoor temp - outdoor temp
Heat loss (BTU/hr) = U-factor * area * delta-T
Season heat loss = heat loss * heating hours
Season cost = season BTU / 1,000,000 * cost per million BTU
This is the conductive heat-loss equation Q = U * A * delta-T. U-factor is the inverse of R-value; lower U-factor means a better-insulating window. ENERGY STAR qualifying windows typically have U-factors around 0.30 or lower depending on climate zone.
Worked example
A 36 by 48 inch window is (36 times 48) / 144 = 12 square feet. With a U-factor of 0.30 and a 40 degree difference (70 inside, 30 outside): heat loss = 0.30 times 12 times 40 = 144.00 BTU per hour. Over 2,400 heating hours that is 345,600 BTU, costing about 6.91 dollars at 20 dollars per million BTU.
Window heat loss notes
- U-factor measures how fast heat conducts through the whole window assembly; lower is better.
- ENERGY STAR window U-factor thresholds vary by climate zone, often around 0.27 to 0.40.
- U-factor is the reciprocal of R-value: a U-factor of 0.25 equals an R-value of 4.
- The temperature difference (delta-T) drives instantaneous loss; cold snaps sharply raise it.
- This calculator covers conductive loss only, not air leakage or solar gain.
Window Heat Loss Calculator: frequently asked questions
What is a good U-factor for a window?
Lower is better. ENERGY STAR qualifying windows generally have U-factors around 0.30 or below, with the exact threshold depending on your climate zone. A U-factor of 0.30 corresponds to an R-value of about 3.3.
How do I calculate heat loss through a window?
Multiply the U-factor by the window area in square feet and by the indoor-outdoor temperature difference in degrees Fahrenheit. The result is the heat loss in BTU per hour. This is the equation Q equals U times A times delta-T.
What is the difference between U-factor and R-value?
U-factor measures the rate of heat transfer (lower is better), while R-value measures resistance to heat transfer (higher is better). They are reciprocals: U-factor equals 1 divided by R-value.
Does this include air leakage?
No. This calculator estimates conductive heat loss through the glass and frame using the U-factor. Air leakage around the window (infiltration) and solar heat gain are separate effects not included here.
Sources and methodology
- U.S. ENERGY STAR: Windows, Doors and Skylights (U-factor guidance).
- The heat-loss equation Q = U * A * delta-T is a standard heat-transfer relationship; 144 square inches per square foot is a fixed unit conversion.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.