The formula
opening = leaf + 2 × lining + 2 × clearance
Where it comes from
The opening is never the leaf. You have to add the lining on both sides and across the top, the clearance that lets it be plumbed — without it there is no way to hang a door straight in a wall that never is — and, at the bottom, the gap left so the leaf does not catch on the floor. With a standard 72.5 cm leaf and a 3 cm lining, the opening comes to 80.5: eight centimetres more than the door measures.
How to work it out by hand
- Add the lining thickness to both sides of the leaf
- Add the fitting clearance on each side
- At the top add lining and clearance; at the bottom only the floor gap
- Check that the existing opening gives that measurement
What is worth knowing
Standard leaf widths step up in increments of ten centimetres: 62.5, 72.5 and 82.5 are the three common ones in housing. An accessible bathroom needs 82.5 as a minimum, because a wheelchair will not pass through 72.5 once the frame is in. And the gap at the bottom is not an oversight: besides stopping the leaf catching, that gap is the air return path when mechanical ventilation or an extractor hood is running, and without it doors are hard to close and whistle. If the floor level is going to rise with new flooring, count it before ordering the door: trimming a leaf afterwards ruins the edge.