How do I measure an alcove or an internal opening accurately?

Internal measurements are where fitted work goes wrong, because an alcove is a shape rather than a number. Chimney breasts lean, plaster bellies out at skirting height, and reveals taper front to back. A shelf cut to one measurement will either jam solid or leave a gap, and which one it does is decided before you pick up a saw.

What you'll need

• Tape measure
• Pencil and paper for a sketch
• Spirit level (optional)
• Laser distance measure (optional)

Step by step

1. Get the tape into the corner properly.Push the tape hard into the corner and add the width of the case, which is printed on the housing for exactly this purpose. Bending the blade round into the corner and estimating is the single most common internal measuring error and it always reads short.
2. Measure the width at three heights.Near the top, at mid height and near the bottom. Write all three down. Never average them and never assume the middle one is representative — you need to know the spread, not a summary of it.
3. Measure the depth front to back as well.For a reveal, a window opening or a deep alcove, take the width at the front face and again at the back. Reveals commonly taper, and a panel cut to the front dimension will not go in.
4. Measure both diagonals.Corner to opposite corner, both ways. Equal diagonals mean the opening is square; unequal means it is a parallelogram, and the difference tells you how much shape you will have to absorb with a scribe or a packer.
5. Check plumb and level too.A level on each side and across the head. An alcove can have equal widths at every height and still lean, which matters the moment you stand a cabinet in it.
6. Choose the right number for the job.Something that must slide into the opening — a shelf, a cabinet, a blind inside the recess — takes the smallest measurement, minus a working clearance. Something that fills the opening and will be scribed or caulked — a panel, a worktop, an infill — takes the largest.
7. Decide your clearance deliberately.A painted timber shelf in a plastered alcove wants two or three millimetres to spare; a carcass you have to manoeuvre in wants more. Zero clearance in an out-of-square opening means the piece binds diagonally and cannot be pushed home at all.
8. Sketch the opening with all the numbers on it.A small drawing with six widths, two depths, two diagonals and which end is high. It takes a minute and it is what lets you decide, at the bench, where to hide the discrepancy.

Tips

• A folding rule or a telescopic rod gives a genuinely rigid internal measurement with no hook and no case allowance, which is why joiners still carry one.
• A laser measure with the fold-out foot is excellent here — it measures from the pivot in the corner, so the case allowance is handled for you.
• If the widest and narrowest readings differ by more than about 10mm, plan on a scribed filler strip rather than a tighter cut. No amount of care makes one straight edge fit a curved wall.
• Take the measurements again just before cutting, not just when ordering. Plastering, tiling and skirting all change an opening after it has been measured.

Common mistakes

• Taking one measurement across the middle — It tells you nothing about the top and bottom, which are usually where the tight point is. This is the reason so many alcove shelves have to be trimmed on site.
• Bending the tape into the corner instead of adding the case — The blade curves, and the curve always reads short. Cutting to that figure leaves a visible gap at both ends.
• Using the largest figure for something that has to fit inside — It will bind before it is home. For an internal fit the tightest point governs, always.
• Ignoring the diagonals — An out-of-square opening cannot be filled by a rectangle. Finding that out at the bench is cheap; finding it out with the piece jammed halfway in is not.

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