Window U-Factor to R-Value Calculator
Windows are rated by U-factor, the rate at which heat passes through the whole assembly, while insulation is usually described by R-value, its resistance to heat flow. The two are exact reciprocals: R equals one divided by U. This calculator converts a window U-factor to its R-value, and also estimates the conductive heat loss through the window from its area and the inside-to-outside temperature difference using the standard heat transfer equation. Enter the U-factor from the window's NFRC label. The heat loss figure covers conduction only, not air leakage or solar heat gain.
U-factor and R-value formula
R-value = 1 / U-factor
Conductive heat loss = U-factor * area * temperature difference
Loss per square foot = U-factor * temperature difference
R-value is the exact reciprocal of U-factor. The conductive heat loss is the standard Q = U x A x deltaT relationship, giving the rate of heat passing through the glazing by conduction.
Notes on window ratings
- Lower U-factor means better insulation; higher R-value means the same thing expressed inversely.
- Take the U-factor from the window's NFRC label, which rates the whole assembly.
- ENERGY STAR sets maximum U-factors that vary by climate zone.
- This heat loss covers conduction only, not air leakage or solar heat gain.
- The conversion is exact arithmetic; the heat loss depends on the temperature difference you enter.
Window U-factor: frequently asked questions
What is the relationship between U-factor and R-value?
U-factor and R-value are reciprocals: R = 1 / U and U = 1 / R. U-factor measures how readily heat passes through, so lower is better for insulation. R-value measures resistance to heat flow, so higher is better. A window with a U-factor of 0.25 has an R-value of 4.0.
What is a window U-factor?
U-factor is the rate of heat transfer through a window assembly, in BTU per hour per square foot per degree Fahrenheit. Windows are rated by U-factor on the National Fenestration Rating Council (NFRC) label. ENERGY STAR sets maximum U-factors that vary by climate zone.
Why are windows rated by U-factor rather than R-value?
Walls and insulation are usually rated by R-value, but windows are rated by U-factor because the whole assembly (glass, frame, spacers, gas fill) is measured together. The NFRC reports U-factor for the entire window, which is why window comparisons use U-factor, not R-value.
How do I estimate heat loss through a window?
Conductive heat loss equals U-factor times area times the temperature difference between inside and outside: Q = U x A x deltaT, in BTU per hour. This calculator computes that loss from your U-factor, window area, and temperature difference. It covers conduction, not air leakage or solar gain.
What U-factor should I look for?
Lower is better for keeping heat in. ENERGY STAR maximum U-factors depend on the climate zone and are published on the ENERGY STAR website. Northern zones call for lower U-factors than southern zones. Check the current ENERGY STAR criteria for your region before buying.
Official sources
- U.S. Department of Energy, updating or replacing windows: energy.gov.
- ENERGY STAR, residential windows criteria: energystar.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.