How to measure a door opening correctly
Measuring is the part where a mistake is the most expensive to fix. A slab made to the wrong size can't be returned or reworked — it was made specifically for you.
The good news is that measuring isn't hard. You need a tape measure, a pencil, a sheet of paper, and twenty minutes per apartment.
What you'll need
A tape measure — a metal one, not a soft tailor's tape: it sags and adds a few millimeters of error. A pencil and paper, because there will be more numbers than you'd expect. A level, or any level app on your phone, to check whether the door jambs are vertical.
It's easier with two people if the opening is tall: holding the tape steady at two meters on your own is awkward.
The main rule: not one point, but three
Walls in a real apartment are rarely perfectly even. An opening that measures 800 mm at the bottom might measure 815 mm at the top. If you measure it in one spot and order based on that number, the door either won't fit or will sit with a crooked gap.
So every dimension is measured at three points, and you take the smallest one.
Step 1. Width
Measure the distance between the left and right jambs:
- at the bottom, about 20 cm from the floor;
- at mid-height;
- at the top, about 20 cm from the lintel.
Write down all three numbers. For example: 812, 806, 819. The working width of the opening is 806 mm — the smallest of the three.
Step 2. Height
The same way, from the floor to the top lintel, at three points: near the left jamb, in the middle, near the right jamb.
And here's the detail people get wrong most often.
Measure from the finished floor. If the laminate or tile isn't laid yet, add the whole future floor build-up to your measurement: screed, underlay, the finish surface itself. The gap between "measured off the concrete" and "floor's now laid" can easily reach 5–8 cm — and a door that was supposed to sit flush ends up hitting empty space or hitting the floor.
If the floor isn't in yet, write down both numbers separately: height off the concrete is this much, planned floor build-up is this much.
Step 3. Wall thickness
Measure the depth of the opening — from one wall face to the other. Also at two or three spots: in older buildings, a wall can be "drunk" (uneven through its depth).
This number decides whether you need a jamb extension — a board that adds to the frame's depth. A telescopic frame comes in 80, 100 and 120 mm and adjusts to wall thickness within certain limits. If the wall is thicker, an extension is added: 100, 150 or 200 mm.
Don't skip this measurement. The jamb extension is the item most often "forgotten" in a preliminary quote, only to show up on the invoice later and spoil the impression.
Step 4. Checking the geometry
Hold a level against the left and right jamb. If they're noticeably out of plumb, tell your manager — you may need a telescopic frame instead of a flush one: a telescopic frame forgives unevenness, while a flush frame needs a perfectly flat wall, since the slab has to sit level with it.
While you're at it, check the lintel above: if it sags or slopes, that's worth mentioning too.
How to write down the result
For each opening, you should end up with something like this:
- Room: bedroom
- Width: 812 / 806 / 819 → working 806
- Height: 2035 / 2030 / 2038 → working 2030, floor already finished
- Wall thickness: 175 mm
- Jambs: even
- Opens: into the room, handle on the right
That last line matters as much as the numbers. Which side the door opens on — left, right, or sliding — is set at the factory, and it can't be changed after production. Stand with your back to the hinges: if the door swings toward you and the handle is on the right, it's a right-hand door.
Now the most important part: opening size ≠ door size
A slab is always smaller than the opening. There's a gap left between the frame and the wall for expanding foam and leveling — without it, the frame can't be set level.
Exactly how much to leave depends on the frame type, wall thickness, and how even your opening is. So you shouldn't convert your measurements into a door size yourself. Send your manager exactly what you recorded — sizing the door for you is our job, and our responsibility for the result.
If you've bought doors before and are used to calculating it yourself, send us your raw measurements alongside your own calculation anyway. Two numbers side by side beat one.
Which sizes come with no surcharge
Our grid runs in 50 mm steps: width from 400 to 900 mm, height from 1800 to 2100 mm.
Within that range, there's no surcharge for size at all — an 850 mm-wide slab costs exactly the same as the most common 800 mm one. This is the difference that most often brings customers to us from competitors. Most catalogs treat a handful of fixed widths as standard, and if your opening falls between them, you're offered either resizing the opening or paying as for a custom order.
Outside the grid, we make to order: width of 950 or 1000 mm, height from 2150 to 2300 mm — plus 20% on the slab price. A custom-size frame and jamb extension are priced the same way.
Timeline: 45 days from deposit for a standard size, 60 days for a custom one.
Special cases
- An opening wider than a meter. The grid stops at 900 mm; we make custom sizes up to 1000 mm. Anything wider can't be covered by a single slab — it needs a different solution. Send us the sizes and we'll find an option that fits your opening.
- Sliding doors. Here a fourth measurement joins the usual three: the free stretch of wall to the side, where the slab slides to. It needs to be at least as wide as the slab itself, with no outlets, switches or radiators on it. Measure it right away — it decides whether a sliding door is even possible.
- An arched opening. A frame is rectangular, so an arch either gets boxed in to a rectangle or is left without a door. It's better to decide this before the finishing work, not after.
- An opening you're still planning to change. If you're planning to widen a passage or wall up an extra door, do that first, then measure. Measuring "how it will be" almost always ends up different from how it turns out.
Five mistakes we see most often
- One measurement instead of three. A wall that looks even isn't the same as a wall that measures even.
- Height measured off the subfloor. The most expensive mistake of all, because it surfaces at the very end of the renovation.
- A forgotten wall-thickness measurement. It later turns out a jamb extension is needed, and the quote changes.
- No opening side specified. Or specified as "whatever's convenient" — and convenient turns out to mean different things to the customer and the installer.
- Rough estimates instead of measurements. "About eighty by two hundred" isn't a measurement. Millimeters matter here: 806 and 810 give a different result in the quote.
What to do next
Walk through your apartment with a tape measure and record every opening using the scheme above — it takes about twenty minutes. Send us the list, and you'll get a line-item quote back within one business day.
If any of your openings turned out to be non-standard — wider than 900 mm or taller than 2100 mm — check the non-standard sizes page: it covers what we make beyond the grid and on what terms.
If you'd like to understand the construction first — frames, architraves, hardware fitting — all of that is on the specifications page.