Watch when it drips. If it leaks while the sink sits unused, the source is the pressurized supply side, meaning a supply line or shutoff valve. If it only leaks when water runs down the drain, the source is the drain side, like the P-trap or slip-joint washers.
The timing test that pins down the source
There are two water systems under your sink, and they leak at different moments. The supply side is pressurized all the time. Cold and hot supply lines feed up to the faucet from shutoff valves (also called angle stops) on the wall. Because that water is always under pressure, a supply-side leak drips even when nobody has touched the faucet for hours. If you open the cabinet and find water with the sink sitting idle, look at the supply lines and the shutoff valves first.
The drain side carries no pressure. It only holds water for the seconds it takes a sinkful to fall through the P-trap and out. So a drain-side leak shows up only when water is running down the drain and stops soon after. The usual culprits are the P-trap, the slip-joint washers at each curved fitting, or the drain flange/basket where the strainer meets the bottom of the sink. If the cabinet floor is dry until you run the tap, you are looking at the drain assembly.
To identify those exposed parts, compare the sink tailpiece and tubular drain pipe definitions.
A third pattern points somewhere else. If water beads at the base of the faucet on top of the counter and runs down through the mounting holes, that is faucet base seepage, usually a worn gasket or loose mounting nut, not the pipes below. Knowing these three patterns turns a vague wet spot into a short list.
If the cabinet also contains an InSinkErator heated-tank dispenser, its drip, spit and startup checks are a separate appliance task. Record which component releases water and keep clear of wet electrical equipment instead of assuming every wet spot belongs to the ordinary faucet or drain.
How to find the exact leak point
Pipes are sneaky. Water clings to a fitting, runs along the underside of a pipe, then drips off the lowest point, which can be inches from where it escaped. That is why a leak near the back wall can pool at the front of the cabinet. To trace it back to the real source, work in order.
Look at the fixture itself as well as the pipes. If an undermount bowl is visibly dropping or separating, stop using it and see the support and attachment guide rather than tightening drain joints to hold it up.
First, dry everything completely with a towel, including every pipe, valve, and connection. Then lay a few dry paper towels across the cabinet floor so the first drop is easy to spot. Now run the water and watch each joint with a flashlight. Move the beam slowly along the supply lines, the shutoff valves, the slip nuts on the P-trap, and the flange under the basket. The first wet point you see is the source. If nothing drips while the water runs but the cabinet is wet later, go back and check the supply side with the water sitting still, since pressurized leaks can be slow weeps that take time to gather.
The City of Chandler, a metro Phoenix utility, describes the same patient method for a whole-house check: turn off all water, then watch the meter's low-flow indicator, since "if the dial is moving, water is being used or leaking somewhere in the home." The principle is identical under the sink: isolate, watch, and let the water tell you where it goes.
Shut it off first: the immediate step
Before any repair, stop the water so a slow drip cannot become an overnight flood. Reach under the sink and find the shutoff valve on the supply line, usually an oval or football-shaped handle on the wall behind the basin. Turn the handle clockwise until it stops. There is one for hot and one for cold, so close the one feeding the leak, or both if you are unsure. That isolates the faucet without touching the rest of the house.
If the under-sink valve is missing, frozen, or spins without stopping (common with old hardware and hard-water scale, which Phoenix has in volume), shut off the main water valve for the whole home instead. Once the water is off, open the faucet to drain the pressure that is left in the lines. A drain-side leak does not need the supply shut off to stop, since it only runs when you use the sink, but closing the valve while you work keeps the area dry.
If the wet spot is at the shut-off valve or the hose above it, see why an angle stop or supply line leaks for how to tell the stem, the nuts and the line apart.
DIY fixes versus when to call a plumber
Many under-sink leaks are a reasonable Saturday repair. For a drain-side leak, hand-tighten the slip nuts on the P-trap a quarter turn; they often loosen over time. If that does not stop it, take the trap apart and replace the slip-joint washers, which are cheap and sold in any hardware aisle. For a supply-side leak, snug the compression nut at the valve; if the braided line itself is corroded or bulging, replace the supply line entirely, since they are inexpensive and screw on by hand plus a wrench turn. A worn faucet-base gasket is also a standard swap.
Call a plumber when the shutoff valve itself leaks or will not close, when the leak is inside the wall behind the angle stop, when the drain flange is corroded into the sink, or when tightening and washer swaps do not hold. Pinhole or weeping leaks on the rigid pipes upstream of the valve are also a pro job. Skip the chemical drain cleaners while diagnosing; they do nothing for a structural leak and leave caustic standing water that a plumber then has to handle. If you cannot find the source at all, that points to a hidden leak, and our high water bill with no visible leak guide walks through the meter test.
If the leaking sink is old and you are thinking of replacing it in a bathroom remodel, how to pick a bathroom sink lists the faucet, drain and trap details to check first.
Why a small under-sink leak is worth fixing now
A drip under a sink looks minor, but the numbers and the location argue otherwise. EPA WaterSense reports that household leaks waste nearly 1 trillion gallons of water nationwide each year, and the average home loses more than 9,300 gallons annually to leaks, "the amount of water it takes to wash more than 300 loads of laundry." Fixing easy leaks can trim roughly 10 percent off a water bill. A weeping supply line under a sink is exactly the kind of slow loss that adds up unseen.
The bigger risk is the cabinet itself. Under-sink water sits in a dark, enclosed box with a particle-board floor, which is close to ideal conditions for mold growth and water damage. Swollen cabinet bottoms, peeling laminate, and a musty smell all signal that water has been pooling longer than you think. Phoenix hard water makes it worse over the long run: USGS classifies water above 180 mg/L of mineral content as "very hard," and Phoenix tap water runs near the top of that range, so scale builds on valves and washers and shortens the life of every fitting in the cabinet. Catch the drip early, dry the cabinet out fully, and the fix stays small. Wipe the cabinet daily for a few days after a repair and watch for any moisture returning, since a leak that comes back means the fix did not hold. If the same sink also drains slowly, our how to fix a slow-draining sink guide covers that separate problem.
If the faucet is a Moen, see why a Moen bathroom faucet leaks for how Moen sorts under-sink leaks into shut-off valve, supply line and connection hose causes.
A small sensor under the sink can warn you early. See are water leak sensors worth installing.
If the leak follows a new sink install, recheck the steps in how to install a kitchen sink.
Renters can read how long an Arizona landlord has to fix a plumbing leak before sending a repair request.
For a leak on an exposed supply pipe that you can't fix on the spot, temporary ways to stop a leaking pipe lists what works until the plumber arrives.