What size internal door do I need?

Replacement internal doors are sold in a handful of standard sizes that descend from imperial dimensions, and metric equivalents that are close but not identical. A house built before the war may have nothing standard about it at all. The measuring is simple, but the decision about which way to round is the thing that decides whether the job is an hour or an afternoon.

Step by step

1. Measure the existing door, not the opening.Take the door off if it is easier, and measure width and height at three points each, plus the thickness. The old door already accounts for the clearances that were needed, and it tells you what actually fitted.
2. Measure the opening as well, as a cross-check.Between the linings at top, middle and bottom for the width, and from the head to the floor at both sides and the middle. Old openings are frequently out of square by several millimetres, and the smallest measurement is the one that governs.
3. Note the floor covering.The clearance at the bottom has to allow for whatever is going down. A door that clears a bare floor by a few millimetres will drag on a thick carpet, and this is the single most common reason a newly hung door has to come off again.
4. Understand the standard sizes.British internal doors are sold in a short list of imperial-derived sizes — 27, 30 and 33 inches wide are the common domestic widths, with metric equivalents alongside that are a few millimetres different. Thickness is typically 35mm for internal doors and 44mm for fire doors and some heavier designs.
5. Always round up, never down.A door can be trimmed; it cannot be made bigger. Buy the next size up and take the surplus off. How much can safely be removed depends on the door's construction.
6. Check how much the door can lose.A solid timber door can lose a fair amount from each edge. An engineered or hollow-core door has only a narrow solid lipping around a light core, and taking more than that lipping off exposes the core and ruins the door. The manufacturer states a maximum trim — read it before buying.
7. Check the thickness against the existing hardware.A 35mm door will not accept hardware set up for a 44mm one, and vice versa. Latch backset, hinge size and any lock or handle spindle all assume a thickness.
8. Consider a made-to-measure door for an odd opening.If the opening is significantly outside the standard sizes, a bespoke door or a door blank cut to size is often cheaper than the alternative, which is rebuilding the lining. Get a price before assuming it is expensive.
9. Check fire doors separately.If the door is a fire door — a loft conversion, an integral garage, some flats — the size, thickness, certification and permitted trim are all governed by the door's certification, and trimming beyond the stated allowance invalidates it.

Tips

• Buy all the doors for a house at once if you are replacing several. Timber colour, veneer and moulding profiles vary between production batches.
• Measure every opening rather than assuming they match. Houses that look consistent frequently have one narrower door to a cupboard or a bathroom.
• If the opening is badly out of square, the door has to be scribed to it. That is a normal part of the job in an older house, not a sign that you have measured wrongly.

Common mistakes

• Measuring the opening and ordering that size — The door has to be smaller than the opening by the clearance gaps all round. Order to the opening size and there is nothing left to plane.
• Trimming an engineered door beyond its lipping — It exposes the hollow or particle core, which will not hold a hinge screw and cannot be finished neatly. The manufacturer's maximum trim is a real limit.
• Forgetting the floor — Carpet, underlay, LVT and a threshold strip can add a surprising amount. Decide the floor before hanging the door, or leave the bottom oversize until the floor is down.

Questions people ask