A bottle trap is chosen for looks and access; a P-trap for seal depth and price. Both do the same job, and the choice is usually settled by whether the trap is visible.
The bottle trap
A vertical cylinder with a removable cup. It looks deliberate under a wall-hung basin, and the cup unscrews by hand for cleaning, which is genuinely convenient. It takes more vertical space than it looks, and that is what fouls a drawer in a vanity unit.
The P-trap
A U-bend into a horizontal outlet. Cheaper, plainer, and usually deeper in water seal than a bottle trap of the same size — which is the argument for it in a room used rarely.
The space-saving trap
A flattened design that hugs the wall behind a vanity unit. It is the answer when a drawer has to close, and it buys that space partly out of the seal depth. Read the figure rather than assuming.
What to check either way
Inlet thread — 1”1/4 for a basin, 1”1/2 for a sink — outlet diameter, usually 32 or 40 mm, the water seal depth, and whether it comes apart without tools.
Questions people actually ask
Can I fit any trap to any basin?
The threads are standardised, so most fit. What varies is the space under the basin and, in a vanity unit, whether the trap collides with a drawer. That is the measurement to take before ordering.
Why does my new trap smell when the old one did not?
Compare the water seal depths. A slimmer replacement often has a shallower seal, and in a bathroom used a few times a week that difference is exactly what shows up as a smell.
Last reviewed 5 September 2026