Metric U-Value to Imperial U-Factor
To convert a metric U-value to a US U-factor, multiply it by 0.17611: a European Uw of 1.30 W/(m2.K) is a U-factor of 0.23 Btu/(h.ft2.F). To go back, multiply the U-factor by 5.678. US code, ENERGY STAR and NFRC read the imperial U-factor; European data sheets, Canadian code and ENERGY STAR Canada read the metric U-value in W/(m2.K). Type either one and the tool converts live, both directions.
W/(m²·K) - European spec sheets, Canadian code
Btu/(h·ft²·°F) - US code & NFRC
Typical European specs
Formula: U-factor = U-value x 0.17611 (and U-value = U-factor x 5.678). European data sheets and Canadian code quote the metric whole-window U-value (Uw); US energy code and NFRC labels use the imperial U-factor. Same physical property, different units - this is the conversion that trips up most import projects in the US and Canada.
Same performance, two unit systems
U-value and U-factor both describe how much heat moves through a whole window. Europe reports it as Uw in W/(m2.K); the US reports the same thing as a U-factor in Btu/(h.ft2.F). The conversion is a single multiplier: U-factor = U-value x 0.17611.
That means a European triple-glazed unit at 0.70 W/(m2.K) is about a 0.12 U-factor - well inside US high-performance territory. A Passive House target of 0.80 W/(m2.K) is roughly a 0.14 U-factor. Knowing the imperial number is how you confirm an imported window will pass your local energy code before you commit to an order. In Canada the code reads the metric U-value directly, so a European Uw compares as-is, and a US-rated U-factor converts back with x 5.678. In both unit systems, a lower U-value is better - and the matching R-value (R = 1 / U) is higher.
The metric note: 1 W/(m2.K) = 0.17611 Btu/(h.ft2.F), and 1 Btu/(h.ft2.F) = 5.678 W/(m2.K). The two factors are reciprocals of each other, so converting there and back always returns the number you started with.
This page is strictly the metric-to-imperial step. If what you need is the resistance figure builders use for insulation, go to the U-factor to R-value converter - it accepts metric input directly. For where these specs actually matter, see my guide to Passive House windows.
How to convert U-value to U-factor: 3 worked examples
- 1
European double glazing to US U-factor
A data sheet lists Uw = 1.30 W/(m2.K). 1.30 x 0.17611 = 0.229, so the window is a U-0.23 window in US terms.
- 2
Passive House target to US U-factor
The Passive House window target is Uw 0.80 W/(m2.K). 0.80 x 0.17611 = 0.141, about a U-factor of 0.14.
- 3
US code limit to metric U-value
A US code table asks for a U-factor of 0.30 or lower. 0.30 x 5.678 = 1.70, so any European window at Uw 1.70 W/(m2.K) or below meets it.
U-value to U-factor chart for windows
| Metric U-value, W/(m2.K) | US U-factor | R-value | Typical window |
|---|---|---|---|
| 2.80 | 0.49 | R-2.0 | Older double-glazed unit |
| 1.70 | 0.30 | R-3.3 | About the US code maximum in most mixed and cold zones |
| 1.61 | 0.28 | R-3.5 | Calgary (NBC zone 7A) prescriptive target - confirm with your building official |
| 1.30 | 0.23 | R-4.4 | European double glazing, Low-E |
| 0.90 | 0.16 | R-6.3 | Triple-glazed aluminum (Reynaers MasterLine 8 is Uw 0.88) |
| 0.80 | 0.14 | R-7.1 | Passive House window target |
| 0.70 | 0.12 | R-8.1 | European triple glazing, high-performance frame |
| 0.60 | 0.11 | R-9.5 | Best aluminum systems (Aluprof MB-86 AERO is Uw 0.60) |
Whole-window values. U-factor = metric U-value x 0.17611, rounded to two decimals; R-value = 1 / U-factor. Lower U is better, higher R is better. The tested value for a specific size and glass comes with the quote.
Common questions
How do you convert metric U-value to imperial U-factor?+
To convert a metric U-value to an imperial U-factor, multiply the U-value in W/(m2.K) by 0.17611 to get the U-factor in Btu/(h.ft2.F). For example, a European U-value of 1.30 equals a U-factor of about 0.23. To go back, multiply the U-factor by 5.678 - 0.30 becomes about 1.70.
Are U-value and U-factor the same thing?+
They measure the same physical property - how much heat passes through the window - just in different units. Europe quotes the metric whole-window U-value (Uw) in W/(m2.K); the US uses the imperial U-factor in Btu/(h.ft2.F). The numbers look different but describe identical performance.
What U-factor does a Passive House window hit?+
A Passive House window targets an installed whole-window U-value of 0.80 W/(m2.K) or lower, which converts to a US U-factor of roughly 0.14. High-performance European triple-glazed windows around 0.70 W/(m2.K) land near 0.12 to 0.14, and the best aluminum systems reach Uw 0.60, about 0.11.
What is a good U-value for windows?+
A good U-value for windows is 1.30 W/(m2.K) or lower for European double glazing and 0.80 or lower for Passive House triple glazing - about 0.23 and 0.14 as US U-factors. US energy code asks for roughly 0.30 or lower in most mixed and cold climate zones, which is about 1.70 W/(m2.K). Lower is better in both unit systems.
Why does this conversion matter for importing European windows?+
European data sheets list performance in metric U-value, but US energy code, ENERGY STAR, and NFRC labels require the imperial U-factor. Converting correctly is how you confirm an imported unit meets your local code before you order - one of the most common spec mix-ups on import projects. In Canada it is the other way around: code and ENERGY STAR Canada read the metric U-value, so the converter also turns a US-rated U-factor back into W/(m2.K).
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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