The leak under the kitchen sink showed up the week after a new strainer went in, and it only appeared when the basin was full. The strainer was not defective. The rubber gasket was installed upside down, so the seal got tighter as the sink emptied and looser as it filled. That is the part most people skip: a basket strainer is three measurements and one gasket orientation away from being completely reliable, and every failure I have seen under a kitchen sink traces back to one of those four things.
Why a kitchen sink uses a strainer and not a stopper
A kitchen sink sees food scraps, grease and a disposal, so it is built around a basket strainer that catches solids in a perforated basket you lift out and empty. A bathroom basin uses a pop-up stopper because its job is to hold water, and the two are not interchangeable: the kitchen opening is wider, the body is deeper to clear a disposal flange, and a kitchen fitting has to survive hot water and food waste. If your kitchen sink currently has a push button stopper screwed into it, someone solved the wrong problem - the right answer for a kitchen is a strainer with a removable basket, and a pop-up stopper belongs in a bathroom.
The three measurements that decide the fit
Measure the sink, not the old part, because the old part may be the wrong size to begin with. The first measurement is the drain opening, the hole cut in the bottom of the basin: on almost every North American kitchen sink it is three and a half inches, but measure it, because a few imported sinks use different sizes. The second is the flange diameter - the ring that sits on the sink floor - which has to be wider than the opening and flat enough to sit on the basin without rocking. The third is the depth of the fitting: the distance from under the flange to the end of the threaded body, which has to be long enough to pass through your sink's thickness and still take the gasket, the friction washer and the locknut. A stainless steel sink is thin, a cast iron or fireclay sink is thick, and a strainer bought for one will not reach through the other. While you are measuring, note the tailpipe size as well: kitchen drains are normally an inch and a half, bathroom drains an inch and a quarter.
Materials, seals and what actually matters
The basket and flange should be stainless - 304 is the grade to look for, and thick beats shiny. Chrome plated zinc looks the part in photos and pits within a couple of winters. The sealing stack under the sink is what keeps water off your cabinet floor, and it has one correct order: plumber's putty or a silicone ring under the flange on the sink side, then the rubber gasket underneath the basin, then the friction or paper washer, then the locknut. Reversing the gasket is the classic mistake, and it produces exactly the symptom in the story above - a leak that appears only under a full basin. Overtightening is the second mistake: it squeezes the gasket out of position and can crack a composite sink, so the locknut only needs to be firm plus a slight turn.
| Part | What to check | Where people go wrong |
|---|---|---|
| Drain opening | 3.5 in on most kitchen sinks - measure the hole itself | Buying the size of the old part |
| Flange | Wider than the opening, and flat on the basin | A flange that rocks on a curved basin |
| Depth | Reaches through the sink and still takes the locknut | Cast iron and fireclay sinks need a deeper body |
| Gasket stack | Putty above, rubber gasket below, friction washer, locknut | Rubber gasket installed upside down |
| Tailpipe | 1.5 in on kitchen drains, 1.25 in on bathroom drains | Buying a bathroom strainer for a kitchen sink |
Replacing it without creating a leak
Start by clearing the cabinet, putting a shallow pan under the trap and shutting off the water. Disconnect the tailpipe and the trap, then undo the locknut and drop the old strainer out from above. Clean the opening thoroughly, inside and out, because a ridge of old putty or food debris will hold the new flange off the sink floor and leak no matter how hard you tighten it. Roll a rope of putty about the thickness of a pencil around the underside of the new flange and press the strainer into the opening, then, from below, fit the rubber gasket, the friction washer and the locknut. Tighten by hand, then a quarter turn with a wrench. Reconnect the tailpipe, run water, and fill the basin to the rim to test the seal properly, since a slow drip only shows up under standing water. Check the joint again after a hot wash, because heat expands the metal and settles the gasket.
When the leak is not the strainer
Not every drop under a kitchen sink comes from the strainer. A leaking tailpipe thread, a slip joint washer installed with the bevel facing the wrong way, a loose trap, the dishwasher or disposal connection, and the sink's own overflow gasket are all common sources, and they all look the same from the outside. Dry everything with a towel, put a sheet of paper under each joint, run the water, and follow the first spot that darkens - that is the joint to fix. If water appears at the back of the cabinet only when the basin is full, it is the overflow or the flange seal rather than the strainer body. If it appears when the disposal runs, check the disposal flange and the dishwasher inlet with the hose clamp.
If the parts above the drain are what you are replacing rather than the strainer, the measuring method is the same but the sizes are not: how to measure a sink drain stopper and 1.25 vs 1.5 inch drains cover the bathroom side.
The bottom line
A kitchen basket strainer is a simple part that fails for four reasons: the wrong size, the wrong material, the gasket upside down, or overtightening. Measure the opening, the flange and the depth before you buy, keep the sealing stack in the right order, and test with a full basin rather than a running tap. If you are replacing the parts above the drain instead, the one we make is our pop-up sink drain stopper with a hair catcher - a bathroom basin part, and we would rather tell you that than sell you the wrong one for a kitchen.
Common questions
What size is a kitchen sink drain opening?
Almost every kitchen sink in North America is drilled for a three and a half inch opening, which is why most basket strainers are sold in that size. Measure your own opening before ordering, because composite and imported sinks occasionally differ.
Can I put a pop-up stopper in a kitchen sink?
Mechanically it will fit most three and a half inch openings, but it is the wrong part for the room. A kitchen sink needs a basket strainer that catches food waste; a pop-up stopper is designed to hold water in a bathroom basin.
Why is my new strainer leaking under a full sink?
The usual cause is the rubber gasket installed upside down, so the seal loosens as the basin fills. Fit the gasket below the basin in the order the instructions show, keep the flange sealed with putty on the sink side, and test with a full basin.
How tight should the strainer locknut be?
Firm by hand plus about a quarter turn with a wrench. More than that squeezes the gasket out of position, strips threads on thin castings and can crack a composite sink - and it will not cure a leak caused by the wrong gasket order.
The same job from the other direction is covered in Bathtub Drain Stopper Sizes and Types (and Why Sink Stoppers Don't Fit).
Related reading
- Sink drain stopper sizes: how to measure yours
- 1.25 vs 1.5 inch sink drains: which one do you have?
- How to clean a kitchen sink drain with a garbage disposal
If you are replacing the parts above the drain rather than the strainer, the one we make is our pop-up sink drain stopper with a hair catcher - a bathroom basin part, and we would rather say that than sell you the wrong one for a kitchen.