U-Factor to R-Value Converter
Type a window U-factor to get its R-value, or the reverse. They are reciprocals - R = 1 / U - so the tool updates both fields live as you type.
Btu/(h·ft²·°F) — lower is better
h·ft²·°F/Btu — higher is better
Typical window values
Formula: R = 1 / U. The two are reciprocals, so a lower U-factor always means a higher R-value. Window R-values look small next to wall insulation because a whole glazed unit resists far less heat than a stud bay - compare windows to windows, not to a wall assembly.
U-factor and R-value, in plain English
U-factor measures how readily a window loses heat: the lower the number, the better it insulates. It is the spec US energy code and NFRC labels use, so it is what plan reviewers look for.
R-value is the inverse - resistance to heat flow, where higher is better. Builders know it from insulation. Since R = 1 / U, the two always move in opposite directions: a U-factor of 0.30 equals an R-value of 3.33, and a triple-glazed 0.15 equals about R-6.7.
If your spec sheet shows a metric whole-window U-value in W/(m2.K) instead, use the metric U-value to U-factor converter first, then come back here. For how glazing choices move your number and your budget, see triple vs double glazing.
Common questions
How do you convert U-factor to R-value?+
Take the reciprocal: R-value = 1 / U-factor. A window with a U-factor of 0.30 has an R-value of about 3.33 (1 / 0.30). To go the other way, U-factor = 1 / R-value. Both use US imperial units, so no unit conversion is needed - just the reciprocal.
Is a lower U-factor better?+
Yes. U-factor measures how much heat escapes through the window, so lower is better. Because R-value is the inverse, a lower U-factor always corresponds to a higher R-value (more resistance to heat flow). High-performance European triple-glazed units reach U-factors of about 0.14 to 0.20.
What is a good U-factor for windows in the US?+
It depends on your climate zone and energy code, but ENERGY STAR generally looks for roughly 0.30 or lower in colder zones. Premium European systems comfortably beat that - double Low-E glazing lands near 0.26 to 0.30, and triple glazing reaches 0.14 to 0.20.
Why are window R-values so low compared to wall insulation?+
A whole glazed unit resists far less heat than an insulated stud bay, so window R-values (roughly R-3 to R-7) look small next to wall R-values (R-13 and up). That is normal - compare windows to other windows, not to a wall assembly.
Speccing high-performance European windows?
Get a live landed-cost range for triple-glazed European systems, or send me your schedule for a line-item DDP quote.