Why Is My Kitchen Faucet Leaking Underneath the Sink, and How Do I Fix It?

If you’ve opened the cabinet and found a puddle, a damp particle-board base, or water dripping from the faucet shank, you’re dealing with a kitchen faucet leaking underneath the deck — and the good news is that this is one of the most fixable plumbing problems in the house. In most kitchens, an under-sink leak is a $5–$25 parts job and 30–60 minutes of work, not a plumber’s call-out. The trick is correctly identifying the source before you start turning wrenches, because « water under the sink » can come from at least five different places, and the fix for each is completely different.

Below I’ll walk you through exactly how to find the leak, what causes each type, how to fix it step by step, and when it’s genuinely smarter to replace the faucet instead of repairing it. I’ve repaired and pressure-tested hundreds of kitchen faucet assemblies, and the diagnostic order here is the same one I use on a service bench.

How do I find exactly where my kitchen faucet is leaking from underneath?

Dry everything completely, lay down a sheet of paper towel under each connection, then turn the water on and watch which towel gets wet first. Water travels and drips, so the lowest wet spot is rarely the actual source — the paper-towel test isolates the real origin within a couple of minutes.

Work top to bottom and check these points in order, because that’s the order of likelihood for an under-sink leak:

• The supply-line nuts — where the braided hoses screw onto the angle stops and onto the faucet tailpieces. The single most common culprit.
• The faucet mounting nut / shank — water running down the faucet body from a worn O-ring above the deck or a bad base gasket.
• The pull-down sprayer hose — the quick-connect fitting or a pinhole where the hose rubs the cabinet.
• The angle stop valve itself — leaking from its packing nut, usually only when you turn it.
• Condensation or a drain leak — not a faucet problem at all, but easily mistaken for one.

One distinction matters a lot: is the leak constant, or only when the water runs? A leak that only appears while the tap is on points to the supply pressure side — supply lines, faucet O-rings, or the sprayer hose. A leak that’s there even with the faucet off, but the stops open, points to the angle stops or the connections themselves under static pressure. A puddle that appears only after you run the sink for a while, with no faucet involvement, is usually the drain or a tailpiece — a different repair entirely.

Why is water dripping from the faucet supply lines under my sink?

Because the connection nut is either under-tightened, over-tightened (which deforms the rubber washer), or the braided line itself has corroded or burst. Supply-line connections account for more under-sink leaks than every other cause combined, and they’re the easiest to fix.

Each braided supply line has two ends: one screws onto the angle stop (the shut-off valve coming out of the wall or cabinet floor), and one screws onto the faucet’s threaded tailpiece. Both ends have a built-in rubber or nylon washer that does the sealing — which means you should never wrap these threads in plumber’s tape. Tape on a washered compression-style fitting actually prevents a proper seal.

To fix a leaking supply connection:

1. Close both angle stops (turn clockwise until they stop). If the stops are seized or there are none, shut the main water valve.
2. Open the faucet to release pressure, then put a bucket and towel underneath.
3. Snug the leaking nut about a quarter to half turn with a wrench — hand-tight plus a nudge. Don’t crank it; brass strips and washers crush.
4. Turn the water back on and re-run the paper-towel test.
5. If it still weeps, unthread the line completely and inspect the washer. A flattened, cracked, or missing washer means replace the whole braided line — they’re a few dollars and you should replace both at once.

If the braided sleeve shows rust bloom, kinks, or a bulge, replace it immediately regardless of whether it’s leaking yet. A burst supply line under household pressure can flood a kitchen in minutes. Quality stainless-braided lines are rated for years of service, but the cheap ones that ship with budget faucets are the first thing I swap. The same diagnostic logic applies to bathroom setups too — if you want the parallel walkthrough, our guide on why your bathroom faucet is leaking and how to stop it covers the supply-side checks in the same order.

What if the leak is coming from the faucet body or base, not the connections?

If water is running down the outside of the faucet shank and dripping off the mounting nut, the leak is starting above the counter — usually a worn O-ring under the spout or a failed gasket where the faucet base meets the deck — and gravity carries it underneath. You’ll often see this as a clean drip off the lowest point of the faucet body with bone-dry supply lines.

There are two versions of this:

Leak from above the deck running down: On pull-down and single-handle faucets, the spout rotates on a set of O-rings. When those O-rings dry out and crack, water seeps down inside the faucet column and exits at the base, then drips under the sink. The fix is to replace the spout O-rings — pop off the spout, roll the old rings off, lubricate the new ones with silicone faucet grease, and reassemble. A full O-ring set runs $5–$15. The procedure is mechanically identical across most faucet families; our step-by-step on RO faucet O-ring replacement shows the technique clearly, and it transfers directly to a kitchen spout.

Leak at the base gasket / loose mounting nut: If the whole faucet wobbles, the mounting nut under the deck has backed off, breaking the seal of the base gasket. Water from around the spout (or a sprayer dock) then runs under the deck and drips below. Tighten the mounting nut from underneath, and if the rubber/foam base gasket is flattened or torn, replace it. This is extremely common on faucets that have been installed for 5+ years.

Why does my pull-down sprayer hose leak under the sink?

Because the quick-connect fitting where the hose joins the faucet supply has worked loose or lost its gasket, or the hose has developed a pinhole where it chafes against the cabinet, the basket strainer, or its own counterweight. Sprayer-hose leaks are sneaky — they often only drip while you’re actively using the spray function, then stop, so the cabinet seems randomly wet.

Check the hose along its full length while the water runs and the spray is engaged. Look for:

• A loose quick-connect collar — push it fully together until it clicks; replace the rubber gasket inside if it’s torn.
• A worn spot where the hose rubs metal — reroute it and add a foam sleeve, or replace the hose.
• A leaking counterweight clip area where constant flexing fatigues the hose.

Most manufacturers sell the hose-and-spray-head as a replacement assembly, and on a faucet still under warranty they’ll often send it free. If your faucet is a known leaker brand, our breakdown of a leaking Glacier Bay kitchen faucet walks through the sprayer-hose and cartridge fixes in detail and is worth a read even if you own a different brand — the parts are nearly universal.

How do I tell a faucet leak from a drain leak or condensation?

Run only the faucet (don’t fill the basin) and watch underneath — if it stays dry until you let water drain, the problem is your drain or P-trap, not the faucet. If the cabinet is damp only on humid days with cold pipes, it’s condensation. This 60-second test saves people from replacing a perfectly good faucet.

The drain side has its own seals — slip-nut washers on the P-trap and the basket strainer gasket — that fail with age and look exactly like a faucet leak from across a dark cabinet. Tightening the slip nuts hand-tight plus a quarter turn fixes most of them.

Should I repair my kitchen faucet or just replace it?

Repair it if the leak is a supply line, O-ring, gasket, or sprayer hose — those are cheap, fast fixes that restore the faucet to like-new. Replace it if the faucet body is cracked, the finish is peeling, the cartridge is a discontinued part, or you’ve already replaced O-rings twice in two years. As a rule of thumb, if repair parts cost more than about 50% of a comparable new faucet, replace.

The single biggest predictor of a long-lived, leak-free faucet is the valve inside it. Faucets built around a ceramic disc cartridge dramatically outlast old rubber-washer compression designs, because two polished ceramic discs seal against each other with almost no wear. If you’re shopping for a replacement, start with our roundup of the best kitchen faucets with ceramic valves — it’s the upgrade that prevents the next leak.

When you do replace, you’ll first need to get the old one out cleanly without cracking the supply stubs or the sink deck. The full procedure — including how to free a seized mounting nut — is in our guide on how to remove old taps from a sink without wrecking the plumbing underneath.

What tools and parts do I need to fix an under-sink faucet leak?

You need surprisingly little — most under-sink leak repairs use the same short list of tools. Have these ready before you shut the water off:

• Adjustable wrench and/or a basin wrench (essential for the high, hard-to-reach mounting and supply nuts)
• Channel-lock pliers
• Replacement braided stainless supply lines (buy the correct length and end fittings)
• Faucet O-ring/cartridge repair kit for your specific model
• Silicone faucet grease (never petroleum-based — it destroys rubber seals)
• Bucket, towels, and a flashlight or headlamp
• Plumber’s tape — only for non-washered threaded joints, never on compression/braided connections

Always match repair parts to your faucet’s brand and model number, usually stamped under the spout or printed in the manual. Generic O-rings that are even half a millimeter off will leak again within weeks.

How do I stop my kitchen faucet from leaking underneath again?

Replace both supply lines with quality stainless-braided ones, snug every connection to spec rather than gorilla-tightening it, lubricate O-rings with silicone grease at every service, and check the cabinet for dampness once a season. A two-minute seasonal flashlight check catches a $10 supply-line weep before it becomes a warped cabinet and a mold problem.

A few habits that genuinely prevent repeat leaks:

1. Don’t over-tighten. Crushed washers and stripped brass are the #1 cause of leaks that come right back. Hand-tight plus a quarter turn is the target for washered fittings.
2. Upgrade the supply lines when you repair — the lines that come free with budget faucets are the most failure-prone part of the whole assembly.
3. Service O-rings preventively if your water is hard; mineral scale chews through seals faster. Choosing scale-resistant finishes and quality valves pays off long-term.
4. Keep the under-sink cabinet uncluttered so you’ll actually notice a leak early, and so the sprayer hose isn’t chafing against stored bottles.

FAQ

Is a kitchen faucet leaking underneath an emergency?

Usually no — a slow drip can wait a day or two, as long as you close the angle stops to stop the water in the meantime. It becomes urgent only if a supply line has burst or is spraying, or if water has been soaking the cabinet base long enough to risk mold or structural damage. When in doubt, shut the stops and dry everything out.

Can I fix a leaking faucet without turning off the main water supply?

Yes, in most kitchens. There are two angle-stop shut-off valves directly under the sink — one for hot, one for cold. Closing those isolates the faucet so the rest of the house keeps running. Only fall back to the main valve if the angle stops are seized, missing, or are themselves the thing that’s leaking.

Why does my faucet only leak underneath when I turn on the water?

That points to the pressure side of the system: a supply-line connection, a worn internal O-ring, or the pull-down sprayer hose. When the faucet is off, those parts aren’t under flow pressure, so they stay dry; turn the water on and the weak seal weeps. Run the paper-towel test while the water flows to pinpoint which one.

How much does it cost to fix a kitchen faucet leaking underneath?

DIY, almost always $5–$40 in parts — a pair of braided supply lines, an O-ring kit, or a replacement sprayer hose. A plumber’s visit for the same job typically runs $125–$250 depending on your area. Since the diagnosis and repair are genuinely beginner-friendly, this is one of the highest-value DIY plumbing fixes there is.

Should I use plumber’s tape on the supply line connections?

No. Braided supply lines and compression fittings seal with a built-in rubber or nylon washer, and plumber’s tape actually interferes with that seal. Save the tape for threaded pipe joints that don’t have a washer. If a washered connection leaks, the answer is a fresh washer or a new line — not tape.

My faucet is only a year old and already leaking underneath — what now?

Check the warranty before you buy any parts. Most reputable kitchen faucets carry a limited lifetime warranty on the faucet and finish, and manufacturers will often ship a free cartridge, hose, or O-ring kit for a faucet this young. Confirm the leak isn’t simply a supply nut that loosened during installation — that’s a 30-second tightening fix, not a defect.

About the author: This guide was written by the arcorarobinet fixtures team, drawing on hands-on bench testing and field repair of hundreds of kitchen and bathroom faucet assemblies. About arcorarobinet: arcorarobinet designs and supplies kitchen and bathroom faucets, shower systems, and sink fixtures built around ceramic-disc cartridges and stainless-braided supply lines. Our faucets are pressure-tested before shipping and meet standard North American plumbing certifications (cUPC/NSF-style lead-free compliance), and our kitchen faucets are backed by a limited lifetime warranty on function and finish. Always follow your local plumbing code and your faucet’s installation manual.