How to Fix a Leaking Toilet Supply Line

A slow drip from the toilet supply line is one of those repairs that’s easy to ignore — until you find a warped baseboard, a moldy subfloor, or a water bill that’s jumped by $30 a month. The good news is that replacing or repairing a toilet supply line is one of the most beginner-friendly plumbing jobs you can tackle.

A slow drip from the toilet supply line is one of those repairs that’s easy to ignore — until you find a warped baseboard, a moldy subfloor, or a water bill that’s jumped by $30 a month. The good news is that replacing or repairing a toilet supply line is one of the most beginner-friendly plumbing jobs you can tackle. No soldering, no special licenses, and no expensive plumber visit required.

This guide walks you through everything: identifying the source of the leak, understanding the difference between braided and plastic lines, fixing a stubborn compression fitting, repacking a shutoff valve, and doing a full supply line replacement from start to finish.

What You Need

Before you start, gather these supplies. Having everything within arm’s reach saves you from dripping water while you run back to the hardware store.

• Braided stainless steel toilet supply line (3/8 in. x 7/8 in., 12 in.) — The gold standard for durability. Resists kinking and bursting far better than plastic.
• Adjustable slip-joint pliers — For loosening and tightening the compression nut and ballcock nut without scratching the fittings.
• Basin wrench — Useful if your shutoff valve sits awkwardly behind the toilet and your hands can’t reach the nut comfortably.
• Packing string / valve packing cord — Needed if the leak originates at the shutoff valve stem, not the supply line itself.
• Plumber’s grease (silicone grease) — Lubricates the shutoff valve so it turns without cracking. Also helps seat rubber washers.
• White terry cloth towels and a small bucket — For catching residual water when you disconnect the line and keeping the floor dry during the repair.

Step 1: Locate and Identify the Leak Source

Before touching a wrench, spend two minutes confirming exactly where the water is coming from. Wipe the entire supply line and both fittings completely dry with a cloth, then wait 60 seconds and watch closely.

Common leak locations:

• Compression fitting at the shutoff valve: Water beads right where the supply line threads onto the valve. This is the most common failure point.
• Ballcock nut at the fill valve: Water appears at the underside of the toilet tank where the supply line connects. Usually caused by a worn rubber washer inside the nut.
• Along the body of the line: Cracks or pinhole rust spots in an older braided or plastic line. Replacement is the only fix.
• Shutoff valve stem: Water drips from behind the handle, not the supply line connections. This requires repacking the valve stem (covered in Step 4).

Mark the location mentally or with a piece of tape, then move to the appropriate repair section below.

Step 2: Shut Off the Water and Drain the Line

Turn the shutoff valve clockwise until it stops. If the handle won’t turn or feels very stiff, apply penetrating oil and wait 10 minutes before trying again. Never force a corroded shutoff valve — it can snap and leave you with a fully open pipe. If the valve is truly seized, shut off the main water supply instead.

Once the valve is closed, flush the toilet to empty the tank. Hold the handle down until the tank is fully drained. Then place your bucket under the supply line and unscrew the nuts at both ends (valve end first, tank end second). Some water will remain in the line — let it drain into the bucket.

Step 3: Braided vs. Plastic — Choosing the Right Replacement Line

If your existing supply line is the white or gray ribbed plastic type, do yourself a favor and upgrade to braided stainless steel. Here’s why it matters:

Plastic supply lines:

• Brittle after several years of heat/humidity cycling
• Prone to cracking at the compression fitting from overtightening
• Cheaper upfront but higher long-term failure risk
• Cannot be used in warm climates with high ambient temperatures near water heaters

Braided stainless steel lines:

• Flexible inner polymer tube wrapped in corrosion-resistant steel mesh
• Withstand up to 125 PSI in most residential applications
• Resist kinking if the toilet is moved for cleaning
• Typically last 10+ years with no maintenance

Sizing: Measure or inspect your old line before buying. Most residential toilets use a 3/8-inch compression end (valve side) and a 7/8-inch ballcock end (tank side). The length is typically 12 inches, though 16-inch and 20-inch lines are available if your shutoff valve is farther from the tank.

Step 4: Fixing a Compression Fitting Leak (No Full Replacement)

If the line itself is in good condition but the compression fitting at the shutoff valve is dripping, try this before replacing the entire line:

1. Tighten the compression nut one-quarter turn clockwise with slip-joint pliers. Do not overtighten — you can crack the fitting or damage the valve seat.
2. Turn the water back on slowly and watch the fitting for 60 seconds.
3. If it still drips, turn water off, remove the nut completely, and inspect the compression ring (ferrule) inside. A damaged or flattened ferrule must be replaced — you cannot reuse a deformed ferrule.
4. Slide a new ferrule onto the supply line pipe end, reinstall the nut, and tighten firmly.

If the leak persists after a new ferrule, the valve seat itself is pitted and you’ll need a full shutoff valve replacement or a supply line with a pre-attached compression end.

Step 5: Repacking a Leaking Shutoff Valve Stem

A shutoff valve that drips around the stem (behind the handle, not at the supply line connection) needs new packing material. This is a five-minute fix that costs less than $2.

1. Shut off the main water supply.
2. Open the shutoff valve completely to relieve pressure.
3. Using an adjustable wrench, unscrew the packing nut located just below the handle. Turn counterclockwise.
4. Pull the stem out. You’ll see old graphite packing or a rubber washer at the base. Remove all old packing material.
5. Wrap new packing string clockwise around the stem 4–5 times (the direction matters — wrapping with the thread ensures it self-tightens when the valve is closed).
6. Reinstall the stem and packing nut. Tighten snugly but not excessively.
7. Turn the main water back on and test for drips.

Alternatively, if the valve is more than 15 years old or shows significant corrosion, replace the entire shutoff valve. A new 1/4-turn ball valve is far more reliable than an old multi-turn compression valve.

Step 6: Installing the New Supply Line

With the old line removed and the work area dry, installation takes about five minutes:

1. Inspect the fill valve inlet. Look inside the ballcock nut connection at the bottom of the tank. If there’s a rubber washer inside the nut, check that it’s flat and intact. Replace it if it’s cracked or has an impression groove.
2. Thread the ballcock nut onto the fill valve by hand. Turn clockwise until snug. Overtightening cracks the plastic fill valve inlet.
3. Connect the compression end to the shutoff valve. Slide the nut over the valve nipple, then push the compression ring into place. Thread the nut on by hand, then tighten with pliers — one full turn past finger-tight. Do not use Teflon tape on a compression fitting.
4. Check alignment. The line should have a gentle curve — not kinked, not stretched taut. A line under tension will work the fittings loose over time.
5. Turn the shutoff valve back on slowly. Watch both connection points during the first 30 seconds of water flow.
6. Flush the toilet twice. Observe both fittings while the fill valve cycles and the tank refills. Dry the fittings with a cloth afterward and recheck in 10 minutes for any slow seeping.

Step 7: When to Replace the Shutoff Valve Instead

Sometimes the real problem isn’t the supply line at all — it’s the shutoff valve. Signs it’s time for a full valve replacement:

• The valve leaks from the body even after new packing
• The valve won’t fully close, leaving residual water flow
• The handle snapped off or the stem is visibly corroded
• The valve is the old multi-turn compression type (four or more full turns to close)

Upgrading to a 1/4-turn ball valve with an integral supply line escutcheon takes about 30 minutes and costs $15–$25. It’s a worthwhile upgrade that eliminates a chronic leak source and gives you a reliable shutoff in future emergencies.

Preventing Future Leaks

• Replace supply lines every 7–10 years as a matter of maintenance, not just when they fail.
• Avoid overtightening. The single most common cause of immediate supply line leaks after a DIY repair is overtightened compression fittings that crack the ferrule or valve seat.
• Check for hard water buildup annually. White mineral deposits on the fitting indicate hard water that degrades rubber washers faster. Wipe fittings clean and inspect washers each year.
• Don’t use supply lines as a step. Bumping the line with a mop or leaning weight on it fatigues the fittings over time.

Related Reading

• How to Fix a Running Toilet
• How to Replace a Toilet Fill Valve
• How to Unclog a Slow-Draining Sink