How to Fix a Running Toilet
DIY Guide + When to Call a Plumber

That constant hissing or trickling sound from your toilet isn't just annoying — a running toilet can waste 200 gallons of water per day, adding $50–$150 to your monthly water bill. The good news: 90% of running toilet problems can be fixed with a $5–$25 part from any hardware store and no plumbing experience required.

This guide walks you through diagnosing the exact cause of your running toilet and fixing it step by step.

First: How a Toilet Tank Actually Works

Understanding the three main components makes diagnosis much easier:

• Flapper — A rubber seal at the bottom of the tank that opens when you flush and closes to hold water for the next flush. When it deteriorates or warps, water leaks past it continuously.
• Fill valve (ballcock) — Refills the tank after a flush. If it fails, water runs constantly or won't shut off.
• Float — A ball or cup that rises with water level and signals the fill valve to shut off. If the float is set too high or is waterlogged, the tank overfills and water runs into the overflow tube.

Step 1: Diagnose Your Specific Problem

Remove the tank lid and observe for 30 seconds. Look for:

Problem A: Water Is Running Into the Overflow Tube

The overflow tube is the vertical pipe in the center of the tank. If water is flowing over or into its top, your water level is set too high. The fix is adjusting or replacing the float.

Problem B: Water Level Is Fine But Toilet Still Runs

Lift the flapper manually (it's the rubber seal at the tank bottom). If the running stops immediately when you hold the flapper down, your flapper is worn and leaking. This is the most common cause of a running toilet.

Problem C: Fill Valve Runs Continuously

If the water level is correct but you can hear hissing from the fill valve assembly, the fill valve itself is faulty and needs replacement.

Fix #1: Replace the Flapper (Most Common Fix)

Flapper replacement takes about 10 minutes and costs $5–$12. No tools required.

1. Turn off the water supply valve (the knob behind/under the toilet, turn clockwise)
2. Flush to empty the tank
3. Unhook the flapper from the pegs on each side of the overflow tube
4. Disconnect the chain from the flush handle arm
5. Take the old flapper to the hardware store to match the size (or photograph the brand name on the inside of your tank)
6. Hook the new flapper onto the overflow tube pegs
7. Attach the chain to the handle arm with about ½ inch of slack (too much slack and the flapper won't lift fully; too little and it won't seal)
8. Turn the water back on and flush to test

Tip: Universal flappers work on most toilets, but Kohler, American Standard, and Toto have proprietary designs. Match your brand for best results.

Fix #2: Adjust the Float (For Overflow Tube Running)

The float controls water level. Correct water level should be about 1 inch below the top of the overflow tube. How to adjust depends on your float type:

Ball Float (older style — ball on a metal arm)

1. Bend the float arm downward to lower the water level, or turn the adjustment screw at the base of the arm
2. Test by flushing and checking that water stops filling 1 inch below the overflow tube

Cup Float (modern style — cylinder on the fill valve)

1. Pinch the clip on the side of the fill valve and slide the float cup down the valve shaft to lower the water level
2. Some models have a separate adjustment screw at the top of the fill valve

Fix #3: Replace the Fill Valve

Fill valves wear out over 5–10 years. A Fluidmaster 400A (the industry standard) costs about $12 and handles most toilets.

1. Turn off the water supply valve and flush to empty the tank
2. Disconnect the supply line from the bottom of the tank
3. Remove the lock nut on the outside of the tank bottom holding the fill valve in place
4. Pull the old fill valve out
5. Adjust the new fill valve to the correct height (per package instructions)
6. Insert and tighten the lock nut (hand tight + ¼ turn)
7. Reconnect the supply line
8. Attach the refill tube to the overflow tube
9. Turn the water on and set the float to fill 1 inch below the overflow tube

When to Call a Plumber for a Running Toilet

Most running toilet repairs are genuinely DIY-friendly. Call a licensed plumber when:

• You've replaced the flapper and fill valve and it's still running — may indicate a cracked toilet tank, damaged overflow tube, or flush valve seat damage requiring full internal rebuild or toilet replacement
• The toilet rocks or the base leaks — a failing wax ring or damaged toilet flange requires more extensive work
• The toilet doesn't flush completely even after adjusting the chain — may indicate a flush valve seat issue or low water pressure
• Water around the base of the toilet — this isn't a running toilet issue; it's a wax ring or supply line leak requiring a plumber
• You're not comfortable with any of these steps — there's no shame in calling a pro. A toilet repair service call is typically $75–$200, far less than a flooded bathroom