How to Fix a Running Toilet in 3 Steps

A running toilet wastes about 200 gallons a day and costs around $25 to $50 a month on your water bill. The fix is usually $10 in parts and 20 minutes of work, and you don’t need plumbing experience to do it. I’ve fixed this problem in three different houses — twice as a renter, once in my current place — and the diagnosis step is what matters most.

Most running toilets have one of three problems: a worn flapper that’s letting water leak into the bowl, a fill valve that won’t shut off, or a cracked tube inside the tank. The good news is you can figure out which one you have before you buy any parts.

What you’ll need

Tools:

  • Adjustable wrench or pliers
  • Flathead screwdriver (optional, for some flapper types)
  • Bucket or towel
  • Food coloring (for diagnosis)

Materials:

  • Universal flapper kit ($8-15) — most likely culprit
  • Fill valve kit ($15-25) — if flapper doesn’t solve it
  • Nitrile gloves (optional)

Prerequisites:

  • Ability to turn off the water shutoff valve behind the toilet
  • Comfortable lifting and handling the ceramic tank lid (several pounds)

Before you start

Turn off the water supply at the shutoff valve behind the toilet — it’s usually a small oval or football-shaped handle on the wall or floor. Turn it clockwise until it stops. If the valve is stuck or you can’t find it, stop here and call a plumber. Forcing an old shutoff valve can crack it, and then you’ve got a bigger problem.

The tank lid is ceramic and weighs several pounds. Lift it straight up, set it on a towel on the floor in a safe spot. Don’t lean it against anything — one bump and it shatters.

Step 1: Diagnose which part is failing

Before you drive to the hardware store, figure out what’s actually broken.

  1. Remove the tank lid and set it aside safely
  2. Add 10 drops of food coloring to the tank water — don’t flush yet
  3. Wait 30 minutes without using the toilet
  4. Check the bowl water

If the food coloring has migrated into the bowl, your flapper is worn — water is leaking past the rubber seal at the bottom of the tank. This is the most common cause, accounting for about 70% of running toilets.

If the color is still only in the tank but you hear constant hissing or see water trickling into the overflow tube (the tall pipe in the center of the tank), your fill valve isn’t shutting off properly.

Step 2: Replace the flapper (if that’s the problem)

Food coloring being added to toilet tank water for flapper diagnosis test.
Photo by Tima Miroshnichenko on Pexels

This is the easiest fix. I’ve done this repair at least five times.

  1. Turn off the water at the shutoff valve behind the toilet
  2. Flush once to empty the tank
  3. Locate the flapper — it’s the rubber disc at the bottom of the tank, usually black or red, attached to the overflow tube by a chain or rods
  4. Note how it’s attached: either side-mounted on hinge pins, or it slides onto a ring around the overflow tube
  5. Disconnect the old flapper: if it’s on hinge pins, slide the side ears off the pins; if it’s on a ring, lift it straight up and off
  6. Take the old flapper with you to the store to match the size — they come in 2-inch and 3-inch versions, but a universal kit includes both
  7. Install the new flapper: slide it onto the same attachment points and make sure it sits flat on the drain opening with no gaps
  8. Reconnect the chain to the flush lever — leave about 1/2 inch of slack when the flapper is closed
  9. Turn the water back on and let the tank refill

Watch the refill. When the tank is full, the hissing should stop completely. If it doesn’t, the flapper might not be seated correctly — remove it and check for debris around the drain rim, then reseat it.

Step 3: Adjust or replace the fill valve (if flapper didn’t fix it)

If replacing the flapper didn’t solve the problem, the fill valve is the next suspect.

Try adjusting it first:

  1. Turn off the water and flush to empty the tank
  2. Locate the fill valve — it’s the tall assembly on the left side of the tank with a float attached
  3. Find the adjustment mechanism: most have either a screw on top or a clip on the side that controls how high the water level rises
  4. Lower the float by turning the screw counterclockwise one full turn, or sliding the clip down about 1/4 inch
  5. Turn the water back on and let the tank refill — the water should stop about one inch below the top of the overflow tube
  6. Listen for silence when the tank is full

If adjusting doesn’t work or the valve is visibly cracked, replace it:

  1. Turn off the water and flush to empty the tank
  2. Place a bucket under the tank — you’ll have spillage
  3. Locate the locknut under the tank where the fill valve connects (you’ll need to reach under the tank from the outside)
  4. Loosen the locknut with an adjustable wrench — turn counterclockwise
  5. Lift the old fill valve out of the tank
  6. Install the new valve following the kit instructions — usually you insert it through the hole, hand-tighten the locknut from below, then give it a quarter-turn with the wrench (don’t overtighten or you’ll crack the porcelain)
  7. Attach the refill tube to the overflow pipe
  8. Turn the water back on and adjust the float height so water stops one inch below the overflow tube opening

Verify it worked

Person turning off water supply valve located behind toilet on wall.
Photo by Anıl Karakaya on Pexels

After your repair, run this test:

  1. Flush the toilet and let the tank refill completely
  2. Wait 10 minutes — listen for any hissing or trickling sounds
  3. Check the water level in the tank — it should stay constant, not slowly dropping
  4. Lift the tank lid and confirm water isn’t running into the overflow tube

If you still hear running water, double-check that the flapper is seated flat and that the fill valve shuts off when the water reaches the correct level.

Troubleshooting

Problem: Flapper closes but water still runs

Check for mineral deposits or debris on the flush valve seat (the rim where the flapper seals). Turn off the water, empty the tank, and run your finger around the opening — even a grain of sand will break the seal. Clean it with an old toothbrush if needed.

Problem: Chain is tangled or too tight

The chain between the flush lever and the flapper needs about 1/2 inch of slack when the flapper is closed. Too tight and the flapper won’t seal; too loose and it won’t lift when you flush. Adjust it by moving the chain to a different hole on the lever arm.

Problem: New fill valve still won’t shut off

The float may be set too high, or there could be debris in the valve body. Try lowering the float first. If that doesn’t work, turn off the water, remove the valve cap (most twist off counterclockwise), and rinse out any sediment you see inside.

When to call a professional

Stop and call a plumber if:

  • The fix didn’t work after replacing both the flapper and fill valve — this suggests a cracked tank or flush valve, which isn’t a DIY repair
  • Water is leaking from the base of the tank onto the floor — the tank-to-bowl gasket or bolts have failed, and this requires tank removal
  • The shutoff valve won’t turn or is leaking when you try to close it — old valves can break, and that’s a job for someone with pipe-cutting tools
  • Your toilet is 15+ years old — at that point, the plastic and rubber parts are all aging out, and a full replacement ($300-500 installed) might be smarter than chasing multiple repairs

I replaced a flapper in a rental once, and three months later the fill valve failed. The landlord ended up replacing the whole toilet because it was from 1998 and nothing else was worth trusting. Sometimes that’s the right call.

FAQ

How much water does a running toilet waste?

A constantly running toilet wastes 150 to 200 gallons per day depending on the severity of the leak. At typical water rates, that’s $25 to $50 per month added to your bill. A moderately leaking flapper might only waste 20-30 gallons a day, but you’ll still hear the periodic refill cycle every 20 minutes.

Can I use any flapper, or does it need to be a specific brand?

Universal flapper kits work for about 95% of toilets made in the last 30 years. The two main sizes are 2-inch and 3-inch drain openings — a universal kit includes both. If you have an unusual toilet (pressure-assist, or a brand like Toto with a proprietary flush valve), check the manufacturer’s website for the correct part number.

How long does a flapper last?

Flappers typically last 3 to 5 years depending on water quality. If you have hard water with high mineral content, they degrade faster. Chlorine and other water treatment chemicals also break down the rubber over time. If you’ve lived in your house more than five years and never replaced the flapper, it’s probably due.

Is it normal to hear the toilet refill occasionally when no one flushed it?

No — that’s called a phantom flush, and it means your flapper is leaking slowly. It’s not a steady run, but water is escaping past the seal, the tank level drops, and the fill valve kicks in to top it off. Replace the flapper and it’ll stop.

This is the repair that taught me I could fix things in my own house. When I was renting, a landlord quoted me three weeks to send someone out for a running toilet — I bought a $10 flapper kit that afternoon and had it done before dinner. If you can identify which part is broken, you can fix it. For more household fixes that pay off fast, check out how to fix a leaky faucet or diy home repairs that save money.