U-Factor to R-Value Calculator & Converter
Type a window U-factor (US builders often call it the U-value) to calculate its R-value, or go the other way as an R-value to U-value calculator. They are reciprocals - R = 1 / U - so the converter updates live as you type. Got a metric U-value in W/(m2.K) from a European data sheet? Enter it in the third field.
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.
Want to know what a window at this U-factor costs supplied to the US? The estimator prices it in two minutes.
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.
U-value to R-value - same formula
In the US, "U-value" and "U-factor" mean the same imperial number, so converting U-value to R-value is the same reciprocal: R = 1 / U. A European data sheet is the one exception - it quotes a metric U-value in W/(m2.K), which is 5.678 times larger than the imperial figure. The tool handles that: enter the metric number in the third field and it converts to U-factor first, then to R-value. A European 1.30 W/(m2.K) window is a 0.23 U-factor and about R-4.4; a triple-glazed 0.70 is a 0.12 U-factor and about R-8.1.
For a dedicated metric-to-imperial U-factor tool with Passive House benchmarks, use the metric U-value to imperial U-factor converter. For how glazing choices move your number and your budget, see triple vs double glazing.
| U-factor | R-value | Typical window |
|---|---|---|
| 1.00 | R-1.0 | Single pane, clear glass |
| 0.50 | R-2.0 | Older double pane, no coating |
| 0.35 | R-2.9 | Builder-grade double, Low-E |
| 0.30 | R-3.3 | ENERGY STAR double, Low-E |
| 0.25 | R-4.0 | Premium European double |
| 0.20 | R-5.0 | Entry triple glazing |
| 0.15 | R-6.7 | European triple glazing |
| 0.14 | R-7.1 | Passive House class |
Whole-window values, rounded to one decimal. Divide 1 by any U-factor to get its R-value, or 1 by any R-value to get the U-factor.
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.
Is converting U-value to R-value the same thing?+
In a US context, yes - people usually say "U-value" when they mean the imperial U-factor, and R = 1 / U applies exactly as above. The exception is a European spec sheet: those quote a metric U-value in W/(m2.K), which has to be converted to an imperial U-factor first (multiply by 0.17611) before you take the reciprocal. If your number came off a European data sheet, start with the metric U-value to U-factor converter.
What R-value does a triple-glazed window have?+
A high-performance European triple-glazed unit runs a U-factor of about 0.14 to 0.20, which is an R-value of roughly R-5 to R-7. Standard double glazing at 0.30 is about R-3.3, and an old single-pane window at 1.00 is R-1.
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?
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