How to Fix a Leaking Pipe Under the Sink (Before Calling a Plumber)
Quick Answer: How Do You Fix a Leaking Pipe Under the Sink?
Most under-sink leaks happen at a joint — a slip nut on the P-trap or a supply line connection — not a crack in the pipe itself. Dry everything, run water, and find exactly where it wets first, then tighten the slip nut or replace the washer inside it.
💰 DIY cost: $5–20 and 20–40 minutes. Plumber cost: $150–350 including the call-out.
A puddle under the sink looks alarming, but it’s rarely the pipe itself failing — almost always a joint that’s loose or has a worn seal.
What’s Actually Wrong? (Diagnosis)
| Symptom | Likely cause | Go to |
|---|---|---|
| Damp only at a joint connection | Loose slip nut or worn washer | Fix 1 |
| Leaks only when water runs, not at idle | Drain-side joint under pressure | Fix 1 |
| Leaks constantly, even with taps off | Supply line connection | Fix 2 |
| Visible crack or corrosion in the pipe itself | Pipe failure | Call a plumber |
What You’ll Need
- Adjustable wrench or slip-joint pliers — $10–15
- Replacement washers/gaskets — $3–8
- Plumber’s tape — $3
- Bucket and old towels — for catching water and drying joints
- Paper towels — for pinpointing the exact leak location
Fix 1: Tighten or Reseal a Drain Joint 30 MIN · EASY
- Dry every joint and pipe surface completely with a towel.
- Wrap dry paper towels loosely around each connection — the P-trap slip nuts, the trap arm, and the drain tailpiece.
- Run water and check which paper towel gets wet first — that’s your exact leak point.
- Hand-tighten the slip nut at that joint, then snug a quarter turn more with pliers.
- Still leaking? Disassemble that joint, check the internal rubber washer for cracks, and replace it.
Pro Tip: I keep a set of channel-lock pliers just for these, set loose on purpose, because it’s too easy to overtighten plastic slip nuts and crack them. Snug is the goal — a fresh washer does more real work than force ever will. — Marcus
Leak isn’t at a drain joint at all, but constant even with taps off? Check the supply line.
Fix 2: Fix a Supply Line Leak 30 MIN · MEDIUM
- Turn off the shut-off valves under the sink before touching the supply lines.
- Check the connection at both the valve and the faucet — hand-tighten first.
- Wrap the threads in plumber’s tape if it’s a threaded fitting, or replace the compression ring if it’s a compression fitting.
- Turn the water back on slowly and check for leaks at both ends.
Watch for: if the shut-off valve itself is the source of the leak, it likely needs replacing — that’s a slightly bigger job worth calling a plumber for if you’re not comfortable with compression fittings.
When You SHOULD Call a Plumber
- The pipe itself is cracked or corroded, not just a joint.
- The shut-off valve won’t fully close or is seized.
- You’ve tightened and resealed and it still leaks — there may be a bigger alignment issue with the pipe run.
Fair pricing: expect $150–350 for a standard under-sink leak repair.
How to Stop It Happening Again
- Check under-sink connections every few months, especially after any DIY plumbing work.
- Don’t overtighten plastic fittings — it stresses and eventually cracks them.
- Keep a small bucket or leak tray under the P-trap as an early warning system.
FAQ: Leaking Pipe Under the Sink
- Why does my under-sink pipe leak only when I actually run the water?
- Leaks that appear only during use, not when the sink sits idle, point to a supply or drain joint rather than a crack in the pipe itself — usually a loose or worn connection.
- Am I going to need to cut or solder anything for this?
- No, not for the vast majority of under-sink leaks. Slip-nut joints and washers are designed to be hand-assembled and disassembled without any cutting, soldering, or specialty plumbing skill.
The Bottom Line
A leaking pipe under the sink is almost always a $5–20 joint fix, not a $150–350 plumber visit. Dry it, find it, tighten or reseal it.
Keep going: How to Fix a Leaky Faucet · Clogged Kitchen Sink? How to Unblock It · more in our Plumbing fix guides.
