There are two cheap parts that prevent most of what this site is about. A silicone cover catches hair before it enters the pipe. A one-way valve stops air coming back up. Neither needs tools, and together they cost less than a call-out fee.
They do different jobs, so decide which problem you have before you buy. Slow drain = you need the cover. Smell = you need the valve. Both = fit both, and the cover sits on top.
Which measurement actually matters
Covers are sold by outside diameter, and the number that matters is the diameter of the recess the old grate sat in — not the span of the slots, and not the width of the tile cut. Measure across the recess three times: it is rarely perfectly round.
Valves are sold by bore diameter and clear depth. A 40 mm valve in a 50 mm bore will not seal; a 50 mm valve in a 40 mm bore will not fit. Depth is the one people forget, and it is the reason most valves get returned.
Fitting it so it stays fitted
Clean the bore first. Soap scum stops silicone seating, and a valve that sits 2 mm proud will lift the cover and rattle. Press it in evenly with two thumbs rather than hammering one edge.
The cover goes on top of the grate, not under it. If yours sits in the recess, buy one sized to the recess; if it sits over the grate, the outside diameter needs to be 5–10 mm larger than the grate.
The maintenance nobody does
A cover that is never emptied becomes a sealed lid, and the drain starts to pool. Empty it after every shower — four seconds — and rinse the mesh weekly. A valve needs the hair pulled off its lips every month or it stops closing.
Step by step
Each step is one picture. The hotspot marks the part you touch; the caption is the whole instruction.
Recess diameter
Measure the recess, not the slot span.
Bore diameter
Cover for hair. Valve for smell. Both if both.
Valve rim
Push the valve in until the rim sits flush.
Flow test
Test with a full jug, then clean weekly.
What not to do
These are the four ways a ten-minute job turns into a call-out. Every one of them is a red-bordered figure for a reason.
DON'T
Don't force a deep valve into a shallow drain.
Before you close the page
Buy two covers
They cost five dollars and one will go down the drain while you are cleaning it.
Ten-minute job
Measure, clean, press, test. If it takes longer than that, something is the wrong size.
Not a fix for a blockage
A cover prevents hair entering. It will not clear what is already in the pipe.
Related guides
How to Clear a Hair Clog Without Chemicals (Step by Step)
Hair does not dissolve. Chemicals turn it into a jelly that is harder to hook out, which is why the physical route wins every time.
The 5-Minute Weekly Drain Routine That Prevents Everything
Three habits, five minutes a week. Every problem on this site starts with one of them being skipped.
Smell but No Clog: How to Fix a Dry or Broken Drain Seal
The drain runs perfectly and still stinks. That narrows it to one part, and you can test it with a cup of water in under a minute.
Safety
Wear gloves and eye protection. Ventilate the room. Never mix chemical cleaners — bleach plus caustic cleaner releases chlorine gas. Trap contents are grey water and may carry bacteria. If water backs up in more than one drain, or you smell gas, stop and call a licensed plumber.