Window Heat Loss Calculator
A window heat loss calculator estimates how fast heat escapes through a window, using the window's U-factor, its area, and the temperature difference between inside and outside. The U-factor measures how readily a window conducts heat: a lower U-factor means a better insulating window. The rate of heat loss equals the U-factor multiplied by the window area multiplied by the indoor minus outdoor temperature difference, giving an answer in BTU per hour. This tool takes all three inputs and multiplies them to return the heat loss rate. Every value is editable so you can compare a single-pane window (high U-factor) against a modern double or triple-glazed unit, at the design temperature difference for your climate. The temperature difference is the key weather input, and the US National Oceanic and Atmospheric Administration publishes the climate and temperature records that set typical heating-season conditions for your area. Use the result to compare two window upgrades, estimate the heating penalty of adding a large glass wall, or feed a room heat-loss total. Every figure is computed deterministically from the formula shown below, with a worked example that reconciles exactly to the calculator so you can follow each step yourself.
Window heat loss multiplies U-factor, area and temperature difference: Q = U x A x dT. A window with U-factor 0.30, area 30 sq ft and a 40 degF difference loses 360 BTU/hr. A lower U-factor cuts the loss.
Window heat loss formula
Q = U x A x dT
Q = heat loss rate (BTU per hour)
U = window U-factor (BTU per hour per square foot per degree F)
A = window area (square feet)
dT = indoor minus outdoor temperature (degrees F)
A lower U-factor or a smaller temperature difference reduces the heat loss. Triple-glazed windows can reach U-factors near 0.15, less than half the loss of a basic double-glazed unit.
Worked example
A double-glazed window with a U-factor of 0.30 and an area of 30 square feet faces a 40 degree Fahrenheit difference between a 70 degree room and 30 degree outside air.
- U-factor = 0.30 BTU/hr per sq ft per degF
- Area = 30 sq ft, temperature difference = 40 degF
- Heat loss = 0.30 x 30 x 40 = 360 BTU/hr
The window loses 360 BTU per hour under these conditions. These are the calculator's default inputs, so the result above matches the widget exactly.
Window Heat Loss Calculator: frequently asked questions
What is a window U-factor?
U-factor measures how much heat a window conducts per square foot for each degree of temperature difference, in BTU per hour. A lower U-factor means the window insulates better. Single-pane windows are around 1.0, basic double-glazed around 0.30, and high-performance triple-glazed near 0.15. U-factor is the inverse of R-value.
How is U-factor different from R-value?
U-factor and R-value are reciprocals: U equals one divided by R. Windows are rated by U-factor because they conduct heat readily, while insulation is rated by R-value. A window with U-factor 0.30 has an R-value of about 3.3.
What temperature difference should I use?
Use the difference between your indoor set point and the outdoor design temperature for your climate, which is the cold temperature your heating system is sized to handle. The US National Oceanic and Atmospheric Administration publishes the temperature records that inform these design conditions.
Does this include air leakage?
No. This calculator covers conductive heat loss through the glass and frame only. Real windows also lose heat through air leakage around the sash and frame, which depends on the window's air infiltration rating and its condition. Sealing and weatherstripping reduce that separate loss.
How do I cut window heat loss?
Choose windows with a lower U-factor, add storm windows or interior inserts, use insulating cellular shades, and seal air leaks. Reducing the glazed area on the coldest walls also lowers loss. Each of these lowers one of the terms in the U times area times temperature difference formula.
Official sources
- Climate and design temperature records: US National Oceanic and Atmospheric Administration (NOAA). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.