A puddle in the cabinet under the kitchen sink is one of the most common calls I get — and one of the most satisfying to fix, because nine times out of ten it’s a loose connection you can tighten by hand. The trick is to stop mopping and start diagnosing. As a licensed plumber, the very first question I answer is whether the leak is on the drain side or the supply side, because that cuts the suspect list in half instantly.
Step 1: Drain Side or Supply Side?
Dry everything under the sink completely. Then test:
| What you see | Which side | Usual culprits |
|---|---|---|
| Leaks only when water is running | Drain side (not pressurized) | Slip-joint nuts, P-trap, tailpiece, basket strainer |
| Drips constantly, even with the faucet off | Supply side (pressurized) | Shutoff valve, supply line, faucet connection |
| Water in the cabinet but pipes look dry | Above the trap | Faucet base, sprayer hose, dishwasher/disposal connection |
The Drain-Side Leaks (Most Common)
Loose slip-joint nuts. The plastic nuts that join the tailpiece, P-trap, and trap arm loosen over time. Hand-tighten them first — no wrench needed, and don’t overdo it on plastic. This alone fixes the majority of under-sink leaks.
Worn slip washers or a corroded trap. If tightening doesn’t seal it, the nylon/rubber slip washer has hardened or is seated backward. Pull the joint apart, check the washer faces the right way, or just replace the trap. A fresh PVC kit is a few dollars; see our picks for the best P-trap replacement kits. Metal traps that have corroded should be swapped for PVC.
Basket strainer (sink flange) leak. If water drips straight down right under the bowl, the basket strainer’s seal has failed — usually dried-out plumber’s putty or a loose locknut. Re-seat it with fresh putty or silicone; our best kitchen sink basket strainers roundup covers replacements. A good pair of channel-lock pliers makes backing off the big locknut far easier.
The Supply-Side Leaks
A constant drip that’s there even when no one’s run the tap is coming from a pressurized line. Check the compression nuts at the angle stop (shutoff valve) and where the supply lines connect to the faucet. Braided steel lines fail at the crimp and are cheap to replace — see the best braided supply lines. If the shutoff valve weeps at the stem when you turn it, replace it; our best under-sink shut-off valves picks make that an easy upgrade (a quarter-turn valve is worth it).
The Sneaky Ones: Above the Trap
If the pipes look dry but the cabinet is wet, the water is running down from higher up. The two I see most: a faucet base or sprayer hose leaking and dripping down the supply lines, and on disposal-equipped sinks, the dishwasher inlet hose or the discharge tube gasket. If your disposal is involved, our guide on a garbage disposal not working covers the connections to check. Run each fixture separately to isolate the source.
The Paper-Towel Trick
When you can’t see the drip, wrap a dry paper towel around each joint, run the water, and check which towel wets first. It turns “somewhere under here” into an exact joint in about two minutes. If you genuinely can’t find the source, our guide on how to find a water leak goes deeper.
When to Call a Plumber
Slip nuts, P-traps, supply lines, and basket strainers are all weekend-DIY territory. Call a pro if the faucet body itself is leaking, if a shutoff valve is seized or won’t stop weeping, or if you find soft, swollen cabinet floor — that’s water damage that needs drying out and possibly replacing before it spreads.
Frequently Asked Questions
How do I find where my kitchen sink is leaking underneath?
Dry every joint, then run the water and watch. A leak that only appears when water is running is on the drain side (basket strainer, slip-joint nuts, P-trap, or tailpiece). A leak that drips constantly even with the faucet off is on the pressurized supply side (shutoff valve, supply line, or faucet connection). Wrapping a dry paper towel around each joint and finding the wet one pinpoints it.
Why is my sink leaking underneath only when the water runs?
That’s a drain-side leak. The drain isn’t pressurized, so it only leaks while water is flowing through it. The usual suspects are loose slip-joint nuts on the P-trap, a worn slip washer, a failed basket strainer seal, or a cracked tailpiece. Hand-tighten the slip nuts first — that fixes it more often than not.
Can I fix an under-sink leak myself?
Most under-sink leaks are very DIY-friendly. Tightening a slip nut, replacing a PVC P-trap kit, swapping a braided supply line, or re-seating a basket strainer with fresh plumber’s putty are all hand-tool jobs. The main exceptions are a leaking faucet body or a shutoff valve whose stem packing weeps — those are a bit more involved.
Why is there water in my cabinet but the pipes look dry?
Two common causes: the leak is up at the faucet base or sprayer hose and runs down the supply lines to pool in the cabinet, or it’s the dishwasher inlet/discharge connection on a disposal. Also check that water isn’t tracking in from the countertop or backsplash around the sink rim. Run each fixture separately to isolate it.
What’s the most common cause of a leak under the kitchen sink?
Loose or worn slip-joint connections on the drain — the plastic nuts that hold the P-trap and tailpiece together. They loosen over time or the slip washer hardens. They’re also the easiest to fix: hand-tighten, and if that doesn’t seal it, replace the inexpensive washer or the whole P-trap kit.
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Step-by-step fixes for the 12 most common problems — and exactly when to call a pro. Free and instant, from a licensed plumber. Save hundreds on calls you didn’t need.