U-Factor to R-Value Calculator & Converter
To convert U-factor to R-value, divide 1 by the U-factor: R-value = 1 / U-factor. A window with a U-factor of 0.30 has an R-value of 3.33, and the formula works backwards too - U-factor = 1 / R-value. Type a window U-factor (US builders often call it the U-value) and the calculator updates live, or use it as an R-value to U-value calculator. 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.
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, ENERGY STAR and NFRC labels use, so it is what US plan reviewers look for. Canadian code and ENERGY STAR Canada read the same property as a metric U-value in W/(m2.K) - the calculator goes both ways.
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. Lower U-factor is better; higher R-value is better. Window R-values (roughly R-3 to R-9) look small next to wall insulation (R-13 and up) because a glazed unit resists far less heat than an insulated stud bay - compare windows to other windows.
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.
The metric note in one line: 1 W/(m2.K) = 0.17611 Btu/(h.ft2.F), and 1 Btu/(h.ft2.F) = 5.678 W/(m2.K). Multiply a metric U-value by 0.17611 for the US U-factor, or a U-factor by 5.678 for the metric U-value.
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.
R-value vs U-value (U-factor): what is the difference?
U-factor measures heat transfer through a window, so lower is better; R-value measures resistance to that heat flow, so higher is better. R = 1 / U when the units match: use U-factor in Btu/(h.ft2.F) to get US R-value in h.ft2.F/Btu, and compare whole-window ratings rather than center-of-glass numbers.
| U-factor | Calculation | R-value |
|---|---|---|
| 0.30 | 1 / 0.30 | R-3.3 |
| 0.20 | 1 / 0.20 | R-5.0 |
| 0.15 | 1 / 0.15 | R-6.7 |
What is the R-value of a window?
Modern window R-values are roughly R-3 to R-9, depending on the frame, glass and size. The chart below shows older single-pane windows near R-1, premium double glazing around R-4, and triple glazing around R-5 to R-7.1. A glass-only R-value does not describe the complete window, and converting a European value does not create an NFRC rating.
How to convert U-factor to R-value: 3 worked examples
- 1
U-factor to R-value
A double-glazed window has a U-factor of 0.30. R-value = 1 / 0.30 = 3.33, so the window is about R-3.3.
- 2
R-value to U-value
A spec calls for an R-5 window. U-factor = 1 / 5 = 0.20, so you need a window rated U-0.20 or lower.
- 3
Metric U-value to R-value
A European data sheet lists Uw = 1.10 W/(m2.K). First convert: 1.10 x 0.17611 = 0.194 U-factor. Then R = 1 / 0.194 = 5.16, so the window is about R-5.2.
R-value to U-value conversion
To convert R-value to U-value, divide 1 by the R-value: U = 1 / R. An R-5 window is U-0.20; an R-4 window is U-0.25. Enter the R-value in the calculator above and it fills in the U-factor and the metric U-value for you. The same reciprocal explains why insulation numbers look so different: an R-30 attic layer works out to U-0.033, far below any window, which is why windows are compared with other windows rather than with walls.
| R-value | Calculation | U-factor | Metric U-value, W/(m2.K) |
|---|---|---|---|
| R-3 | 1 / 3 | 0.33 | 1.89 |
| R-4 | 1 / 4 | 0.25 | 1.42 |
| R-5 | 1 / 5 | 0.20 | 1.14 |
| R-6 | 1 / 6 | 0.17 | 0.95 |
| R-7 | 1 / 7 | 0.14 | 0.81 |
| R-8 | 1 / 8 | 0.13 | 0.71 |
U-factor to R-value chart for windows
| U-factor | R-value | Metric U-value, W/(m2.K) | Typical window |
|---|---|---|---|
| 1.00 | R-1.0 | 5.68 | Single pane, clear glass |
| 0.50 | R-2.0 | 2.84 | Older double pane, no coating |
| 0.35 | R-2.9 | 1.99 | Builder-grade double, Low-E |
| 0.30 | R-3.3 | 1.70 | About the code maximum in most mixed and cold US zones |
| 0.28 | R-3.5 | 1.61 | Calgary (NBC zone 7A) prescriptive target - confirm locally |
| 0.25 | R-4.0 | 1.42 | Premium European double |
| 0.20 | R-5.0 | 1.14 | Entry triple glazing |
| 0.15 | R-6.7 | 0.85 | European triple glazing |
| 0.14 | R-7.1 | 0.80 | Passive House class |
Whole-window values, R-value 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. Metric U-value = U-factor x 5.678. Lower U is better, higher R is better.
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?+
A good window U-factor is 0.30 or lower in most mixed and cold US climates. The 2021 IECC residential prescriptive table generally uses 0.30 in zones 3-8, 0.40 in zone 2 and no tabulated maximum in zone 1; local adoption and exceptions matter. European double Low-E glazing is commonly near 0.22 to 0.30, and triple glazing near 0.13 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 window has an R-value of roughly R-5 to R-7, because it runs a U-factor of about 0.14 to 0.20. Standard double glazing at 0.30 is about R-3.3, and an old single-pane window at 1.00 is R-1. Passive House windows at U-0.14 reach about R-7.1.
How do you calculate U-value from R-value?+
Using matching US units, divide 1 by the R-value: U-factor = 1 / R. A whole window rated R-5 has a U-factor of 0.20. R-values can be added for layers in series along the same heat-flow path, not for glass and frame side by side or for separate window ratings. Use the complete window's rated value, not the glass alone.
How do you convert R-value to U-value?+
Divide 1 by the R-value: U-value = 1 / R. R-4 is U-0.25, R-5 is U-0.20 and R-7 is about U-0.14. In the US the result is an imperial U-factor; multiply it by 5.678 if you need the metric U-value in W/(m2.K) that a European data sheet uses - R-5 is about 1.14 W/(m2.K). Canadian code also reads the metric U-value, so the same x 5.678 step gives you the number a Canadian plan reviewer checks.
What is the difference between R-value and U-value?+
U-value (U-factor) is the rate of heat flow through a window, so lower is better. R-value is the resistance to that heat flow, so higher is better. They are reciprocals - R = 1 / U and U = 1 / R - so a window can be described either way. US energy code and NFRC labels use U-factor for windows; R-value is the insulation language builders use for walls and roofs.
Which unit does Canadian building code use for windows?+
Canadian code reads windows in metric: the whole-window U-value in W/(m2.K), or an Energy Rating (ER) where the code allows it. Targets depend on the climate zone - for example, the prescriptive target in Calgary (NBC zone 7A) is a U-value of 1.61 W/(m2.K), about 0.28 as an imperial U-factor, and in Toronto (climate zone 5) it is 1.84, about 0.32. Treat these as information: provincial codes and local amendments differ, so confirm the target with your building official.
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