U-Value to U-Factor Converter
European data sheets quote a metric U-value in W/(m2.K); US code and NFRC use the imperial U-factor. Type either one and the tool converts live, both directions.
W/(m²·K) — European spec sheets
Btu/(h·ft²·°F) — US code & NFRC
Typical European specs
Formula: U-factor = U-value × 0.17611 (and U-value = U-factor × 5.678). European data sheets 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 US import projects.
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.
Once you have the U-factor, you can convert it to an R-value too. For where these specs actually matter, see my guide to Passive House windows.
Common questions
How do you convert metric U-value to imperial U-factor?+
Multiply the metric U-value in W/(m2.K) by 0.17611 to get the imperial 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.
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?+
Passive House glazing typically targets a metric U-value around 0.80 W/(m2.K) or lower, which converts to a U-factor of roughly 0.14. Standard European triple glazing near 0.70 W/(m2.K) lands around 0.12 to 0.14.
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.
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