The energy consultant needs a U-factor for the permit set by Friday. What you have is a European datasheet that says "Uw = 1.2 W/(m2K), Ug = 1.0, g = 53%, Rw = 36 dB", a size of 1230 x 1480 and a weight of 58. None of those numbers can go on a US form as written, and two of them look like numbers you know but mean something slightly different.
My own offers go out in inches, pounds and U-factor. But the system datasheets, glass certificates and test reports behind them stay metric, and so does every quote you get straight from a European factory. Here is how to read one line by line without guessing - and where the two free converters on this site save you the arithmetic.
The cheat sheet
| On the European sheet | Unit | US equivalent | How to convert |
|---|---|---|---|
| Uw (whole window) | W/(m2K) | U-factor | x 0.17611 |
| Ug (glass center) | W/(m2K) | Center-of-glass U-factor | x 0.17611 - not the code number |
| Uf (frame only) | W/(m2K) | No direct code field | Background only |
| g-value | % or 0.xx | Close to center-of-glass SHGC | Same scale, but not whole-window |
| Rw | dB | Close to STC | Different test curve - do not swap blindly |
| Psi (spacer) | W/(mK) | "Warm edge" spacer | Lower is better, no US field |
| Width x height | mm | inches | / 25.4 |
| Weight | kg | lbs | x 2.2046 |
| Area | m2 | sq ft | x 10.764 |
Everything else in this post is the detail behind those nine rows.
Uw is the number that matters - Ug is the trap
A European sheet usually lists three U-values. Uw is the whole window: frame, spacer and glass together. That is the one that maps to the US U-factor in the IECC tables. Ug is the center of the glass only, and Uf is the frame profile only.
The common mistake is converting the prettiest number. A triple-glazed unit might show Ug = 0.5. Multiply by 0.17611 and you get 0.09 - a figure no window on the market reaches as a whole unit. The Uw on the same sheet will sit closer to 0.8, which converts to about 0.14. That is the honest number, and it is still excellent.
For reference, the systems I quote land between 0.22 and 0.32 double-glazed and between 0.13 and 0.19 triple-glazed, whole unit. If a conversion comes out far below that range, check whether you picked up Ug by accident. The good U-factor guide covers what those numbers mean against code, and the climate zone breakdown gives the target for your county.
The conversion itself is one multiplier - U-factor = Uw x 0.17611 - and the metric U-value to US U-factor converter runs it both ways, so you can also check what a US spec of 0.25 means to a European factory (about 1.42).
g-value is almost SHGC - but not quite
The g-value is the share of solar energy the glass lets through, on the same 0-1 scale as the US solar heat gain coefficient. A g of 53% reads as 0.53. The difference is scope: the g-value is usually measured on the glass alone, while the SHGC on an NFRC label is for the whole window. The frame blocks sun, so the whole-window SHGC comes out lower than the center-of-glass g.
In practice that means the g-value is a conservative stand-in if you need to stay under an SHGC cap in a hot climate - but ask for a whole-window figure before you put it on a compliance form.
Rw is not STC
Rw and STC both describe how much sound a window stops, in decibels, and for windows they often land within a few points of each other. They are still different tests with different frequency curves. If a spec calls for STC 35, a sheet that says Rw 36 is encouraging, not proof. Glass build-up drives most of the result - the soundproof windows guide shows which configurations reach which range.
Dimensions, glass build-up and weight
Sizes on a European quote are in millimeters, width first, and they are almost always the outside frame dimension - not the rough opening. Divide by 25.4. Leave your installation gap on top of that; the rough opening guide walks through the clearances.
Glass is written as a build-up, outside to inside: 4/16/4 means 4 mm glass, a 16 mm cavity, 4 mm glass - 24 mm total, just under 1 inch. A triple 4/14/4/14/4 is 40 mm, about 1 9/16 inch. Letters after the numbers describe the panes and the gas: "Ar" is argon fill, "ESG" is tempered, "VSG 33.1" is laminated.
Weight comes in kilograms per position. Multiply by 2.2046. It matters more than people expect - a tall aluminum tilt-turn sash can be heavier than one person should lift into a second-floor opening.
Two more things to check on every drawing. European shop drawings are often drawn from the inside, while US elevations are drawn from the outside, so confirm the handing on each position before you sign. And performance classes like "air class 4" or "watertightness E750" come from European test standards. E750 means the sample held water at 750 Pa (about 15.7 psf), but a NAFS Performance Grade is a different test sequence, so the class does not transfer directly. The NFRC and NAFS guide explains what a plan reviewer may ask for instead.
One line, translated
Here is an illustrative tilt-turn position from a metric sheet, converted:
| Field | Metric | US |
|---|---|---|
| Size | 1230 x 1480 mm | 48 7/16 x 58 1/4 in |
| Area | 1.82 m2 | 19.6 sq ft |
| Uw | 1.2 W/(m2K) | U-factor 0.21 |
| Ug | 1.0 W/(m2K) | Center-of-glass 0.18 (not for the form) |
| g-value | 53% | SHGC under 0.53 whole-window |
| Rw | 36 dB | Roughly STC mid-30s - confirm with a test report |
| Weight | 58 kg | 128 lbs |
Five minutes of work, and the energy consultant gets a number they can use.
Where R-value fits
Some owners and inspectors think in R-value because that is how walls are rated. R-value is simply 1 / U-factor, so a U-factor of 0.21 is about R-4.8 and 0.14 is about R-7.1. That is why even a very good window is still the weakest part of a wall insulated to R-20 or more. The U-factor to R-value converter does the flip, and it helps when you have to explain to a client why triple glazing is worth it in a cold zone - the triple vs double comparison has the rest of that argument.
Next step
If you are holding a metric sheet right now, open the tools page and run the Uw and the R-value conversions for your worst-case window. If you would rather not do this line by line for 30 positions, send me the schedule through the quote form - my quote comes back in inches, pounds and US U-factor per position, ready for the submittal.
FAQ
Is Uw the same as U-factor? Both describe heat loss through the whole window, but Uw is metric (W/(m2K)) and the US U-factor is imperial (Btu/(h.ft2.F)). Multiply Uw by 0.17611 to get the U-factor. Uw 1.3 becomes about 0.23.
How do I convert a European U-value to a US U-factor? Multiply by 0.17611. To go the other way, multiply the US U-factor by 5.678. Use the whole-window Uw, not the glass-only Ug, or the result will look far better than the window really is.
Is g-value the same as SHGC? They use the same 0-1 scale, but the g-value is usually measured on the glass alone, while an NFRC SHGC covers the whole window. The whole-window SHGC is lower, because the frame blocks sun.
Is Rw the same as STC? No. Both are sound ratings in decibels, and for windows they often land within a few points, but the tests use different frequency curves. If a spec calls for STC, ask for an STC figure or a test report.
What does 4/16/4 mean on a window quote? It is the glass build-up in millimeters, outside to inside: 4 mm glass, a 16 mm gas-filled cavity, 4 mm glass. Total 24 mm, just under 1 inch. A triple unit adds a second cavity and a third pane.