You should always measure from the inside of your home. Interior surfaces are usually more consistent and easier to access, which means you’re less likely to encounter uneven materials like stucco, siding, or exterior trims that can distort your readings. Measuring from the inside also lets you reference the structural parts of the opening — the sill, jambs, and head — rather than decorative elements. The only exception is when your supplier specifically requests external “brick-to-brick” dimensions, which is more common in European masonry constructions. When in doubt, take both and clearly label them as inside or outside measurements.

A correctly sized window should be slightly smaller than the measured opening to allow for installation tolerance — the small gap around the frame that’s filled with insulation foam or sealant. For most residential projects, a gap of 10–15 millimeters (⅜–⅝ inch) on each side is ideal. This tolerance ensures the window can be positioned perfectly level and plumb, even if the opening isn’t perfectly square. Too small a gap can cause pressure on the frame and affect smooth operation; too large a gap can lead to poor insulation or water ingress. Most professional installers prefer to have about 5 mm clearance on each side and 5 mm at the top and bottom, but always confirm the manufacturer’s own tolerance recommendations.

Don’t worry — very few openings are perfectly square, especially in older buildings. If your diagonal measurements differ slightly, installers can correct this by shimming the frame during installation. The key is to measure accurately and note the difference. If the diagonals differ by less than 6 mm (¼ inch), it’s generally acceptable. If they differ more, mention it to your supplier or installer — they might recommend making the window slightly smaller or adjusting the wall framing. The goal is to ensure that, after shimming, the window remains level, plumb, and square so it seals properly against air and moisture.

Yes, they do — both in sizing systems and installation methods. In Europe, window measurements are almost always in millimetres (mm) and based on the structural (brick-to-brick) opening. European installers generally leave about 10–15 mm total tolerance for foam and sealing. In North America, measurements are typically in inches and often refer to “nominal” sizes — meaning the advertised 36" × 60" window may actually be slightly smaller to fit a framed rough opening. Also, European windows often open inward (tilt-and-turn or turn-only), while North American ones usually open outward or vertically. When ordering internationally or from metric-based suppliers, always clarify whether dimensions refer to the rough opening size or the actual frame size.

Uneven sills and walls are quite common, particularly in older homes or after renovations. The most important thing is to identify where the variation occurs. Measure the height at the left, center, and right sides to see how much it differs. If the bottom (sill) is sloping, use the shortest height for ordering, since the window can always be shimmed higher during installation but can’t fit if it’s too tall. Similarly, for uneven side jambs, use the smallest width. Minor imperfections (up to about 5 mm) can be corrected during installation, but significant unevenness might need to be leveled before placing the new window.

When windows have external decorative trims, stone cills, or wide plaster reveals, those elements can hide the true size of the structural opening. In this case, remove a small section of trim or measure from the inside of the house where the frame meets the wall. The goal is to capture the actual structural dimension, not just the visible frame. If you can’t safely remove any trim, take both inside and outside measurements and send them to your supplier — professional window manufacturers like KozijnenPro can advise which dimension to use for the final order. Always clearly label your measurements to avoid confusion.

Many homeowners can successfully measure their own openings by following a structured method — using a quality tape measure, taking three width and height readings, checking diagonals, and noting the smallest values. However, if your project involves large windows, uneven masonry, or complex multi-part frames (like bay or corner windows), having a professional take final verification measurements is highly recommended. If you measure yourself, always double-check.

The top mistakes include measuring only once instead of three times, forgetting to check diagonals for squareness, mixing up width and height on order forms, and not accounting for the tolerance gap. Some people also mistakenly measure the old glass or sash size instead of the structural opening. Another common issue is assuming all openings are identical — even two windows side-by-side can vary by a few millimeters. Always label your measurements clearly (for example, “living room left window – width 1200 mm, height 1450 mm”) and verify them before ordering. Consistent documentation prevents confusion later in production.

Accuracy is everything — especially with aluminium systems, which have tight tolerances. Aim to measure to the nearest millimeter (or 1/16 inch). Avoid rounding up, as it’s safer to have a slightly smaller window with shim space than one that doesn’t fit. Double-check your notes and always label which measurement corresponds to which window. The old saying “measure twice, cut once” applies perfectly here: it’s better to spend extra time verifying your numbers than to face an expensive resizing later.