A framer calls me two weeks before a European order ships: the rough opening on the plans says 60 inches, the shop drawing back from the factory says 1500mm, and nobody on site is sure if those are supposed to match or if something's already off. This conversation happens on almost every first-time import, and it's rarely a real problem - it's a units problem that gets treated like a crisis because nobody explained the convention up front.
Frame a bracket-mounted European window's rough opening 1/2 to 3/4 inch larger than the frame on each side, working from the millimeter shop drawing, not a rounded inch size.
European windows are dimensioned in millimeters on a modular coordination system. US construction runs in inches and eighths. The conversion itself is simple arithmetic. What actually causes delays is the second layer underneath it: the rough opening gap convention - how much clearance the frame expects around it - isn't the same practice on both sides of the Atlantic, and if that gets missed, a technically correct opening still doesn't work.
The mm-to-inch conversion, for the sizes you'll actually see
European system providers work in round modular millimeter increments, not odd inch fractions. A few of the sizes that come up constantly on residential jobs:
| Millimeters | Inches (nearest 1/8") |
|---|---|
| 600 mm | 23 5/8" |
| 900 mm | 35 3/8" |
| 1200 mm | 47 1/4" |
| 1350 mm | 53 1/8" |
| 1500 mm | 59" |
| 1800 mm | 70 7/8" |
| 2100 mm | 82 5/8" |
| 2400 mm | 94 1/2" |
Notice these don't land on clean US numbers like 24", 36", or 48" - a 1200mm unit is 47 1/4", not 48". That quarter-inch gap is exactly where a framer working from memory instead of the shop drawing builds an opening that's slightly wrong in both directions. It's small, and it's the single most common measurement mistake I see on a first European order.
The rule that avoids this: the shop drawing is the reference, in millimeters, confirmed against the factory's actual production size - not a rounded inch number pulled from a spec sheet or a previous project. Every project I run gets a shop drawing review before the opening gets framed, specifically so this gets caught on paper instead of on site.
What rough opening clearance does a European window need?
Here's where the real risk sits. In the US, a stick-framed opening is typically built with 1/4" to 3/8" of play for standard tolerance, and the installation practice depends on which mounting method the unit is ordered with - a nailing-flange unit expects the flange to land on sheathing with the WRB lapped over it, while a bracket-mounted unit expects roughly 1/2" to 3/4" of clearance per side to allow for leveling, shimming, and a continuous insulation gap. Most of the systems I source are available either way, so which one applies is a spec decision made up front, not something you can assume from the product line. After the unit is shimmed and centered, that gap gets filled to a 3/8" to 1/2" perimeter insulation space with low-expansion foam - standard high-expansion foam can bow an aluminum profile, so this isn't optional.
The European convention isn't fundamentally different in mechanics - it's the same "frame smaller than opening, shim and seal the gap" logic - but the numbers on the shop drawing are in millimeters, the tolerance is tighter than typical US stick-framing, and the clearance allowance depends on the mounting method actually specified for that order. That combination is why a rough opening built purely from a rounded inch conversion, without reference to the actual shop drawing gap allowance and confirmed mounting method, is the most common cause of a unit arriving that doesn't sit right in the wall.
For sliding and lift-and-slide doors specifically, add one more variable: the sill. A European lift-and-slide sill sits lower and shallower than a typical US patio door threshold, so the rough opening height at the sill line needs its own confirmation - don't assume the same top-of-opening logic that works for a window carries straight down to a floor-level slider.
European window rough opening chart
For planning, here is what the 1/2" to 3/4" per-side clearance means for the common modular sizes - the rough opening is the frame dimension plus 1" to 1 1/2" overall, in width and in height. It is a framing reference for bracket-mounted windows, not a substitute for the shop drawing: nailing-fin units and floor-level sliders follow their own details, and the same logic applies to uPVC, aluminum, wood and aluminum-clad wood frames with the exact allowance confirmed per system. Wood units are built to the opening rather than to catalog sizes, as covered in custom wood windows from Europe.
| Frame size (mm) | Frame size (inches) | Rough opening, bracket-mounted (inches) |
|---|---|---|
| 600 mm | 23 5/8" | 24 5/8" to 25 1/8" |
| 900 mm | 35 3/8" | 36 3/8" to 36 7/8" |
| 1200 mm | 47 1/4" | 48 1/4" to 48 3/4" |
| 1350 mm | 53 1/8" | 54 1/8" to 54 5/8" |
| 1500 mm | 59" | 60" to 60 1/2" |
| 1800 mm | 70 7/8" | 71 7/8" to 72 3/8" |
| 2100 mm | 82 5/8" | 83 5/8" to 84 1/8" |
| 2400 mm | 94 1/2" | 95 1/2" to 96" |
A 1200 x 1500 mm tilt-turn, for example, is a 47 1/4" x 59" frame and wants an opening of roughly 48 1/4"-48 3/4" wide by 60"-60 1/2" high.
How do you measure a rough opening for a European window?
- Measure the width at the top, middle and bottom of the opening, framing to framing. Use the smallest number.
- Measure the height at the left, center and right. Again, use the smallest number.
- Measure both diagonals. Within 1/8" of each other means the opening is square enough to shim.
- Check the sill for level and the jambs for plumb - sliders and doors are the least forgiving.
- Record each opening against its schedule ID, in inches and millimeters, and compare it with the shop drawing frame size plus the clearance for the mounting method.
- On an existing house, survey every opening before production rather than trusting the original plans.
What to actually hand the framer
Skip the "convert everything and hope" approach. What works on every project I coordinate is a one-page opening spec sheet, in this order:
- Opening ID matching the window/door schedule (not just "kitchen slider")
- Width and height in millimeters (the factory number - not a converted approximation)
- Width and height in inches, rounded to the nearest 1/8", labeled clearly as the converted reference, not the primary dimension
- Required clearance per side (from the shop drawing, typically the 1/2"-3/4" range above for bracket-mounted units)
- Sill detail note for any slider or entrance door - flush threshold, ramped, or standard sill pan
- Sign-off line for whoever confirms the as-built opening before the unit ships from the factory
That sheet, not a spreadsheet of raw shop drawing dimensions, is what actually gets read correctly on a job site. Its IDs should match the marks on the window schedule, so every opening traces back to one line of specification.
Measuring a sliding or lift-and-slide door opening specifically? The door version of these steps, with the sill and replacement checks, is in how to measure a sliding glass door.
Who's responsible for the measurement, on a DDP order
This is worth being direct about, because it's the question I get asked on nearly every project once the framer sees mm on a drawing for the first time.
- I coordinate the shop drawing review before production is confirmed - checking that the specified opening sizes match what's buildable, flagging any size that's tight or unusual before it's locked in.
- The framer or GC owns the as-built opening - actually building it to the confirmed dimensions is a site responsibility, same as any other window package.
- Final verification happens before the unit ships, not after it arrives. If there's a real-world opening that doesn't match the confirmed shop drawing, that's the moment to catch it - a site visit or accurate as-built survey before production locks in beats a change order after a container's already at the port.
None of this is unusual for a serious import order - it's the same shop-drawing discipline you'd expect on any custom window package. The difference is just that the reference numbers are in millimeters, and the tolerance for getting the clearance wrong is smaller than typical US stick-framing allows.
FAQ
Do European window sizes ever match standard US rough openings exactly? Occasionally, but don't plan on it. European modular sizes (600mm, 900mm, 1200mm increments) rarely land on round US inch numbers - a 1200mm unit is 47 1/4", not 48". Always frame to the converted shop drawing dimension, not a standard US stock size.
What rough opening clearance should I plan for a European aluminum window? For bracket-mounted units, plan roughly 1/2" to 3/4" of clearance per side for leveling and shimming, then a 3/8" to 1/2" insulation gap after the frame is set and centered. If the order specifies a nailing-flange unit instead, the flange lands on sheathing with the WRB lapped over it - a different detail, so confirm which one applies before framing.
Is the rough opening for a European sliding or lift-and-slide door different from a window? The width and height logic is the same, but check the sill separately - European lift-and-slide sills typically sit lower and shallower than a standard US patio door threshold, so confirm the sill detail against the shop drawing rather than assuming it matches a standard slider install.
How do you measure a rough opening for a window? Measure the width at the top, middle and bottom and the height at the left, center and right, framing to framing, and use the smallest of each. Check that both diagonals are within 1/8" of each other, the sill is level and the jambs are plumb, then compare the result with the shop drawing frame size plus the clearance the mounting method needs.
Who confirms the final rough opening before a European order ships? I coordinate the shop drawing review and flag anything unusual before production is confirmed, but the as-built opening on site is the framer or GC's responsibility, verified against the confirmed drawing before the unit leaves the factory.
Sizing an opening for a real project? Run it through the estimator for a landed cost range, or send me the shop drawing and I'll flag anything that needs a second look before it's framed. For the install-side detail once the unit arrives, see installing bracket-mounted aluminum windows. If you're speccing a modular or prefab build, the tolerance conventions differ again - covered in modular and prefab windows for design-build. And if this is your first European order end to end, the full DDP import guide walks the whole process, shop drawings included. Working on an existing house rather than new framing? Field-verified openings matter even more there - see retrofitting European windows into an existing home.