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Why do I get two different widths measuring the same window?

You measure a window from inside the room, step outside and measure again — and the figures don't match. That is not a mistake. A window opening has several widths, depending on where along the opening you put the tape.

Where does the difference come from?

Three things are usually at play, often at the same time:

• The splay. In many exterior walls, the wall opening is cut wider towards the room than out at the facade, so daylight spreads better into the room. Measured at the inner surface, the opening is therefore wider than measured at the facade surface.
• The frame's position. The window is not necessarily centred in the wall's thickness. If the frame sits right out at the facade, the reveal measurement (the clear opening inside the room) can be noticeably larger than the outer dimension of the frame itself.
• What is actually being measured. The frame's outer dimension, the clear opening (reveal measurement), the glass measurement and the raw wall-hole dimension are four different figures for the same window — and a contractor, a glazier and a bricklayer typically ask for a different one each.

A concrete example: one window, four measurements

Picture an ordinary window in a 30 cm brick exterior wall. Here is how the four measurements can look on exactly the same opening:

• Frame outer dimension: 118 × 138 cm — the window's own physical size, the figure a manufacturer uses to build a replacement window to.
• Glass measurement: typically 8-10 cm smaller in each direction than the frame's outer dimension, because the sash and rebate take up space — here roughly 100 × 120 cm of visible glass.
• Raw wall-hole dimension: 128 × 148 cm — the hole in the wall itself, before the frame is fitted, which a carpenter or bricklayer works from.
• Interior reveal measurement: 138 × 158 cm — the clear opening you see from inside the room, once the splay is included. It is often the largest of the four figures.

Four different figures, one window — a difference of up to 20 cm in width between the smallest and the largest measurement. None of them are wrong. They simply answer four different questions.

What do I do with conflicting measurements?

Don't just write down one figure and throw the other away — note both, and note where each one was taken. That makes it possible to work backwards if someone later asks whether you had the reveal measurement or the frame measurement. A simple rule of thumb:

1. Ordering a new window? Use the raw wall-hole dimension, or whatever measurement the supplier asks for — often the outer opening in the wall, not the interior reveal measurement.
2. Documenting daylight or an escape-route opening? Use the clear opening — the dimension that can actually be used, not the frame's outer dimension.
3. Just need an estimate for a drawing? Pick one principle (say, always the interior reveal measurement) and stick to it throughout the home, so the figures are comparable room by room.

Does a difference of a few centimetres matter in practice?

It depends entirely on what the figure is for. If you are drawing a rough sketch or judging whether a room feels bright, 10-20 cm rarely matters. But if you are ordering a pane of glass, a new frame or an awning that needs to fit precisely, even 2-3 cm is enough for the item to either not fit or need custom adjustment after delivery. The closer a task sits to the actual product being manufactured and installed, the more it matters that everyone knows exactly which of the four measurements has been given — and that it was measured on the very opening the product is going into, not a hole somewhere else in the house that "looks like it's the same".

How the splay affects daylight and view

The splay is not just a source of confusion — it has a function. A wall cut at an angle around a window spreads light better into the room than a straight, right-angled frame would, because it does not create a sharp shadow along the edge of the opening. That is why older houses often have a pronounced splay, while newer buildings with thin, insulated walls have less wall depth to angle with. If you need to judge how much daylight a room actually gets, it is therefore the reveal measurement — the interior, full dimension including the splay — that is the right one to use, not the frame's outer dimension, which underestimates how much sky the room can actually see.

Uneven walls make it worse

If the wall is not quite square, or the wall hole is cut slightly askew, you can end up with two different widths on top of everything else, depending on whether you measure at the top or the bottom of the opening. That is normal in older buildings and not a problem in itself — but it means a single figure never tells the whole story about an old window opening. If the window is being replaced, it is the contractor's job to measure the final hole themselves, right before the new window is ordered — not to trust a measurement taken weeks earlier.

How to avoid mixing up the measurements

• Write down the measuring point alongside the figure: "reveal measurement, interior side" or "frame dimension, exterior".
• Take both measurements if you are unsure what they will be used for afterwards.
• Measure at the same point every time if you are comparing several windows in the same home — so the figures are at least consistent with one another.
• Don't trust an old measurement when a window is actually being ordered. The wall may have settled, and the frame may have shifted slightly over the years.

What HouseSense does

If the same opening gets measured from two sides of the house — for example because two adjoining rooms are both scanned, and both border the same window — the app can end up with two different widths for the same opening. Rather than guessing which figure is correct, HouseSense shows both measurements and asks you to pick the one that fits, in a dialogue called "Two measurements — pick the right one". It's the same decision you would have to make yourself with a tape measure in hand: which point on the opening the figure should apply to.

Which measurement should you give to whom?

1. The window supplier, manufacturing a new window: the raw wall-hole dimension, or whatever measurement they ask for — ask directly if it isn't clear.
2. The glazier, replacing glass in an existing frame: the glass measurement, taken from inside the rebate, not the frame's outer dimension.
3. The architect, or yourself, drawing a plan: the reveal measurement — it gives the most accurate picture of how much opening the room actually has.
4. Anyone else who just needs an estimate: pick one measurement, use it consistently, and write down which one it is.

If the window is actually being ordered, there is only one solution that works regardless of the situation: have a professional measure the final hole right before ordering.