Can I just tighten the nut under the sink to stop it?

Try it once, because a locknut that has worked loose will seal again. Snug it a quarter turn at a time with the basket held still, then run water and check. If it is already snug and still drips, the gasket or the putty has failed and tightening harder only cracks the sink.

Why does it only leak when the sink is full of water?

Standing water pushes down on the seal with real weight, and a tired gasket holds against a trickle but not against that. The fix is the same reseal. Treat it as a failed seal, not as a sink you simply cannot fill.

Should the strainer be sealed with plumber's putty or silicone?

Putty goes under the rim inside the basin, and nothing goes under the rubber gasket below. Silicone works but it glues the strainer in, so the next person has to cut it out. Use putty on a stainless sink and silicone only on stone or composite, where putty can stain.

The strainer in my sink is missing a rubber piece on the bottom. Is that the leak?

Yes, that rubber gasket is the seal, and without it the drain leaks every time water runs. A new gasket is a few dollars and no part of the sink has to come out. Replace the whole strainer only if the body is also corroded.

How long can I leave a strainer leak before it does damage?

Days, not months. Slow drips swell a particleboard cabinet floor, lift the sink base, and grow mold behind the kick panel. Put a pan under it, keep the basin empty, and get it sealed this week.

Sink leaks around or under the drain strainer

Reviewed by Josh Arneson, Handyman and Property ManagerPublished October 12, 2026

A leak that shows up whenever water runs straight down the open drain is at the basket strainer. The strainer is sealed twice: plumber's putty under its rim inside the basin, and a rubber gasket squeezed by a locknut under the sink. Both give out, and a plumber pulls the strainer, cleans the sink, and re-seals it in about an hour. Replace the whole assembly instead when the body is cracked, corroded, or missing its gasket, because a new seal on a pitted body leaks again.

Tell a strainer leak from a trap leak in five minutes

Dry everything. Wipe the underside of the sink, the big locknut on the strainer, the tailpiece, and every nut on the trap.
Lay a dry paper towel on the cabinet floor under the drain.
Run cold water straight down the open drain for 30 seconds. Watch where the strainer comes through the bottom of the sink.
Run your fingertip around the locknut and the top of the tailpiece. Wet metal there is the strainer.
Now plug the basin, fill it, and pull the stopper. Watch the same spot again.
Check the paper towel. Drips in the middle of the towel with the strainer dry point lower down.
Shut the faucet off and leave the sink unused for an hour, then look again.

If the first water appears at the strainer body or the locknut, the putty or the gasket has failed, and that is the repair below. If the strainer stays dry and water shows at a slip joint or the trap, the strainer is fine. If water keeps coming with the faucet off and the drain dry, the leak is on the supply side, and both stop valves under the sink get shut now.

Repair or replace

Situation	What to do
Drips only while water runs down the open drain, strainer body wet	Repair. Pull the strainer, reseal with fresh putty and a new gasket.
Drips only after you drain a full basin	Repair. The seal is failing under the weight of standing water. Same reseal.
Rubber gasket missing, flattened, or torn, strainer body sound	Repair. The gasket and locknut cost a few dollars.
Strainer body cracked, pitted, or corroded	Replace the assembly. A new gasket on a pitted body leaks again.
Locknut spins with no resistance or the threads are stripped	Replace the assembly.
Drain opening in the sink is rusted through or dished	Replace the sink. A seal needs a flat, sound rim to bite on.
Water on the sink deck around the rim, dry underneath	Repair. Only the putty on top has failed.
A garbage disposal hangs on that drain, not a basket strainer	Different part. The mounting ring and its gasket get re-seated, and a leak from the disposal body means a new unit.

What it costs

Runhome contractor estimates for resealing or replacing a sink basket strainer ran from $130 to $360 as of October 2026, including the trip and assessment.

Option	Range
Fit a strainer you already bought	$130 to $315
Reseal the existing strainer with a new gasket	$155 to $315
Replace the whole basket strainer assembly	$170 to $360

Can I do it myself?

Yes, on a plain basket strainer with no disposal, if you can get both hands under the sink. The job is: loosen the slip nut and drop the tailpiece, back the big locknut off, push the strainer up and out, scrape the old putty off the sink, roll fresh putty under the new rim, then fit the gasket, the friction ring, and the locknut from below and tighten while a helper holds the basket still from inside the basin. A spud wrench or large channel locks and about an hour cover it.

Hire someone if the strainer spins instead of tightening, if the locknut is seized, or if the drain is cramped behind a disposal and other pipes, because forcing a seized nut cracks a porcelain or composite sink.

What a contractor will ask you

Have these ready and you skip the back-and-forth.

A wide photo of the sink from above showing the basin, the drain, and the counter, plus one of the drain from inside the cabinet.
The age of the sink, even as a guess.
Whether the strainer is leaking, damaged, or missing a part, or whether you are adding one to an empty hole.
When it drips: only with the basin full and then drained, or also with water running straight down the drain.
Where the water shows: on the sink deck, or as drips inside the cabinet.
Whether a garbage disposal is attached to that drain or it is a plain basket strainer and tailpiece.
The sink material: stainless steel, porcelain or enamel, or composite.
Whether the strainer body and locknut are reachable by hand or cramped behind pipes.
Whether you want it resealed or the whole assembly replaced, and whether you already bought the part.

Common questions

Can I just tighten the nut under the sink to stop it?: Try it once, because a locknut that has worked loose will seal again. Snug it a quarter turn at a time with the basket held still, then run water and check. If it is already snug and still drips, the gasket or the putty has failed and tightening harder only cracks the sink.
Why does it only leak when the sink is full of water?: Standing water pushes down on the seal with real weight, and a tired gasket holds against a trickle but not against that. The fix is the same reseal. Treat it as a failed seal, not as a sink you simply cannot fill.
Should the strainer be sealed with plumber's putty or silicone?: Putty goes under the rim inside the basin, and nothing goes under the rubber gasket below. Silicone works but it glues the strainer in, so the next person has to cut it out. Use putty on a stainless sink and silicone only on stone or composite, where putty can stain.
The strainer in my sink is missing a rubber piece on the bottom. Is that the leak?: Yes, that rubber gasket is the seal, and without it the drain leaks every time water runs. A new gasket is a few dollars and no part of the sink has to come out. Replace the whole strainer only if the body is also corroded.
How long can I leave a strainer leak before it does damage?: Days, not months. Slow drips swell a particleboard cabinet floor, lift the sink base, and grow mold behind the kick panel. Put a pan under it, keep the basin empty, and get it sealed this week.