How do I measure for and fit a replacement toilet seat?

Toilet seats are sold as universal and are not. What is genuinely standardised is the distance between the fixing holes, which is why almost any seat will bolt on. What is not standardised is the shape and size of the pan, which is why so many replacement seats sit with ceramic showing at the sides or a lip hanging over the front. The measurement drawing shows exactly which three dimensions to take.

What you'll need

• A tape measure or a steel rule
• The new seat, with its fittings
• A cloth and bathroom cleaner
• Penetrating oil, for corroded old fixings
• A junior hacksaw, as a last resort on a seized bolt

Step by step

1. Measure A — the hole centres.Lift the old seat off if you can, or measure round it. Take the distance from the centre of one fixing hole to the centre of the other, across the back of the pan. This is nearly always a standard figure and it is what decides whether a seat will physically bolt on at all.
2. Measure B — the length.From the line joining the two fixing holes, straight down the centre, to the furthest point at the front of the pan. This is what decides whether the seat overhangs or falls short.
3. Measure C — the width.The widest point across the bowl, which is usually about a third of the way back from the front rather than at the middle.
4. Measure the pan, never the old seat.The old seat may itself be the wrong size — that is often why it was never comfortable — and old seats warp and spread. Every measurement comes off the ceramic.
5. Identify the shape.Round or O-shaped, D-shaped with a straight back and flattened front, square or rectangular, or a traditional elongated oval. The measurements decide what fits; the shape decides whether it looks right. A seat that fits but is the wrong outline overhangs at the front or leaves a rim of ceramic showing at the sides.
6. Check what kind of fixing holes you have.Holes that pass right through the ceramic take a conventional bottom-fix bolt with a nut underneath. Blind holes that only go part way need a top-fix fitting. And on a back-to-wall or wall-hung pan, the space behind and beneath is closed in, so a top-fix seat is the only option regardless of what the holes look like.
7. Get the old seat off.Lift the hinge covers at the back and undo the nuts or wing nuts underneath. Old plastic ones spin rather than undo — grip the bolt head from above and turn the nut. A corroded metal bolt may need penetrating oil, or as a last resort cutting through with a junior hacksaw blade slid between the seat hinge and the pan, with the ceramic protected by a strip of card.
8. Clean the pan surface completely.The area under the hinges is the least-cleaned part of any toilet. Clean and dry it before the new seat goes on — it will be another ten years before it is exposed again.
9. Fit the seat loosely first and check the position.Get both bolts through with their washers and nuts started, then set the seat so it is central, square and closes cleanly into the bowl. Only then tighten. A seat tightened before it is positioned always ends up slightly skewed.
10. Tighten by hand until it stops moving, and no further.The fittings are plastic and the pan is china. Tighten until the seat is firm and will not shift when you push it sideways. Over-tightening strips the plastic thread or, on a metal fitting, cracks the ceramic.
11. Check the closed position and the front overhang.Sit the seat down. It should meet the rim all the way round, and the front should not project beyond the front of the pan. If it does, the length was measured wrong and the seat is the wrong size — it will get knocked and it will loosen constantly.

Tips

• Buy a seat with stainless steel or good-quality fittings rather than the cheapest plastic ones. The fittings are what fail, not the seat.
• Quick-release hinges, which let the seat lift off with a button press, make cleaning around the hinges genuinely easier and cost very little extra.
• If you cannot find the old pan's brand, take the three measurements and the shape to a merchant. Nobody needs the brand.

Common mistakes

• Measuring the old seat instead of the pan — The old seat may itself be the wrong size, and old seats warp. Every dimension comes off the ceramic.
• Buying on hole centres alone — Almost every seat matches the hole centres. It is the length and the shape that decide whether it fits the pan, and those are what people skip.
• Tightening metal fittings hard into a china pan — China cracks under point load. Firm by hand is correct; a spanner is not.