How to Fix a Leaking Faucet Supply Line

A leaking faucet supply line—that flexible or rigid tube running from your shutoff valve to the faucet—is one of those small problems that turns into big water damage if you ignore it. Water damage under a sink spreads fast, rots cabinet wood, and creates mold conditions before you realize what's happening. The good news: most leaks are fixable in under an hour with basic tools and often without calling a plumber. The key is identifying whether you're dealing with a simple connection that's come loose, a worn ferrule that's lost its seal, or a line that's actually punctured. This guide walks you through diagnosing the leak and executing the repair.

1. Stop the Water First. Locate the shutoff valve under the sink—it's the small lever or knob on the line coming from the wall or floor. Turn it clockwise until it stops. If you can't locate it or it won't turn, shut off the main water valve for the house. Turn on the faucet to release any remaining pressure in the line.
2. Find the Exact Source. Grab a towel and dry the area completely. Wipe down both supply lines (hot and cold) and the shutoff valves. Look carefully at where the leak is coming from: at the connection nut where the line meets the shutoff valve, at the connection point at the faucet base, along the tube itself, or from a pinhole. Mark the exact spot with your finger.
3. Snug the Valve Connection. Position an adjustable wrench on the connection nut (the large hex nut where the supply line screws onto the shutoff valve). Hold the valve body steady with your other hand or a second wrench. Turn the connection nut clockwise one-half to three-quarter turn. Don't over-tighten—you'll crack the fitting. Turn the shutoff valve back on and watch for drips.
4. Tighten the Faucet End. If the leak persists or comes from the faucet end, repeat the process at the faucet base. Most faucets have a hex nut where the supply line threads in. Tighten it gently with your wrench. If you can't reach it due to cabinet obstruction, you may need a basin wrench—a specialized tool with a long handle designed for this exact situation.
5. Swap the Seal Washer. If tightening the nuts didn't work, the rubber or metal ferrule inside the fitting has failed. Turn off the water again, unscrew the connection nut completely with your wrench, and pull the supply line away from the valve. You'll see a small rubber or metal ring (the ferrule) at the end of the line. Remove it and replace it with an identical new ferrule from your hardware store. Reattach the line and tighten the nut firmly but not violently.
6. Install a New Line. If you see a crack in the line itself or a pinhole spray, the line cannot be sealed—it must be replaced. Turn off water, unscrew both connection nuts (at shutoff valve and faucet), and remove the old line. Take it to the hardware store to match the length and thread type. Install the new line hand-tight first, then use your wrench to snug each connection nut. Turn water back on and check.
7. Verify the Fix Works. Turn on the shutoff valve fully and run the faucet for 10 seconds. Look under the sink while water is flowing and for another 30 seconds after you turn the faucet off. Watch for any drips or spray at the connection points. Leave a towel under the connections for an hour, then check it dry.