1. Centre distance, for anything wall-mounted
From the middle of one wall outlet to the middle of the other. Not edge to edge — that is the measurement that buys the wrong tap.
Continental European plumbing settled on 150 mm for bath and shower mixers, and most of what you will find is built for it. Where your wall is 140 or 165 mm, eccentric connectors bridge the difference, and they are usually supplied.
2. Tap hole diameter, and the distance below it
The hole in the basin or worktop is 35 mm on essentially every modern single-lever mixer; some older ceramics are 32 mm.
Then the measurement people forget: from that hole down to the isolating valves. A tap made for a thin ceramic basin often has 30 cm tails, and a thick worktop over a deep vanity unit can put the valves 45 cm away.
3. The toilet, in three numbers, with the seat off
Hinge hole spacing, centre to centre. Length, from the centre line of those holes to the front edge of the pan. Width, at the widest point.
The seat has to be off. Every measurement taken with it on is a measurement of the seat you already have.
4. The space under the basin
From the underside of the waste down to the floor or the shelf, and from the waste back to the wall. This is what decides whether a bottle trap fits, whether a vanity drawer will close, and whether a tall click-clack waste plus a trap will foul the unit.
The one almost everybody takes wrong
Centre distance, measured between the inside edges of the two outlets instead of between their centres. It gives a number roughly 20 mm too small, it looks plausible, and it is the commonest reason a bar mixer arrives and does not fit.
Write them down where you will find them
A photograph of a tape across each measurement, kept on your phone, answers almost every question a listing will ask.
Questions people actually ask
Do I need to measure if I am buying the same model again?
No — that is the one case where it is settled. Everything else, including a different model from the same maker, is a new set of measurements.
What if a listing does not publish the measurement I need?
That is itself information. A maker who publishes spout reach, tail length and cartridge diameter is telling you something about the product before you read the numbers, and one who publishes none of them is telling you something too.
Last reviewed 5 September 2026
