How to Fix a Toilet That Keeps Running

A running toilet is one of the most common — and most wasteful — household plumbing problems. That quiet hissing or trickle you hear coming from the bathroom isn’t just annoying; it can waste 200 to 1,000 gallons of water every single day.

A running toilet is one of the most common — and most wasteful — household plumbing problems. That quiet hissing or trickle you hear coming from the bathroom isn’t just annoying; it can waste 200 to 1,000 gallons of water every single day. Over the course of a month, that’s a water bill increase of $50–$200 for a problem that typically costs under $25 and 30 minutes to fix.

The good news is that nearly every running toilet boils down to one of four causes, and all four are well within DIY territory. You don’t need to be a plumber to fix a running toilet — you just need to know where to look and what to replace. This guide walks you through the complete diagnosis and repair process.

What You Need

Gather these tools and parts before you start:

• Toilet Flapper Replacement — the most common fix; buy a universal or model-specific flapper
• Fill Valve Replacement Kit — for hissing, cycling, or ghost-flushing issues
• Toilet Repair Kit — complete kit with flapper, fill valve, and flush valve assembly
• Adjustable Wrench — for removing the fill valve lock nut
• Food Coloring — for the dye test to confirm a flapper leak
• Sponge and Bucket — for emptying the tank before working

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Understanding How a Toilet Works

Before diving into repairs, it helps to understand what’s actually happening inside that tank. Lift the toilet tank lid and set it safely aside. What you’ll see:

The flapper is the rubber seal at the bottom of the tank. When you flush, it lifts to let water rush into the bowl. After the flush, it falls back down and creates a seal so the tank can refill. If the flapper is warped, cracked, or minerally crusted, it won’t seal completely, and water will constantly seep into the bowl.

The fill valve is the tall mechanism, usually on the left side. It opens automatically when the water level drops (after a flush) and closes when the tank is full. A faulty fill valve can run continuously, cycle on and off every few minutes, or cause a hissing sound.

The float is connected to the fill valve and rises with the water level to tell the valve when to shut off. On older toilets it’s a large ball on a metal arm. On modern fill valves, it’s a cylindrical cup that rides up and down the valve shaft. If the float is set too high, water rises into and down the overflow tube, causing continuous running.

The overflow tube is the vertical pipe in the center of the tank. It’s a safety drain — if the water level gets too high, it flows down the overflow tube directly into the bowl rather than overflowing the tank. If the water level is set above the top of the overflow tube, you’ll hear constant running.

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Step 1: Do the Dye Test

The dye test is the fastest way to confirm whether your flapper is the problem. Drop 10–15 drops of food coloring (or a dye tablet) into the tank. Wait 15 minutes without flushing. If colored water appears in the toilet bowl, your flapper has a leak and needs to be replaced.

You can also do a quick manual test: reach into the tank and press down firmly on the rubber flapper with your finger. If the running sound stops immediately when you apply pressure, you’ve confirmed the flapper isn’t sealing.

If the dye test comes back negative (no color in the bowl), the issue is with the fill valve or float — not the flapper.

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Step 2: Replace the Flapper

Replacing a toilet flapper is one of the easiest plumbing repairs there is. The whole job takes about 10 minutes and costs $5–$15.

Turn off the water supply. The shut-off valve is on the wall behind and below the toilet. Turn it clockwise until it stops. Flush the toilet to drain the tank.

Remove the old flapper. The flapper connects to two pegs on either side of the flush valve (the large seat at the bottom of the tank). Simply unhook the ears from the pegs. Then unhook the flapper chain from the flush handle arm. The flapper will pull free.

Note the flapper type before buying a replacement. Take the old flapper to the hardware store or search the toilet model number (usually stamped inside the tank) to find the right match. Most universal toilet flappers fit the majority of toilet brands and styles, but certain brands — particularly American Standard and Kohler — may require brand-specific flappers for a proper seal.

Install the new flapper. Hook the ears of the new flapper onto the flush valve pegs and attach the chain to the flush handle arm. The chain should have just enough slack that it doesn’t pull the flapper open when the toilet is at rest, but not so much slack that it can get caught under the flapper seal (about 1/2 inch of slack is right).

Turn the water back on. Let the tank fill completely. Flush once and watch the flapper fall and seal. Check for running — if it’s stopped, the job is done.

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Step 3: Adjust or Replace the Float

If replacing the flapper didn’t fix the running, check the water level in the tank. The correct water level is approximately 1 inch below the top of the overflow tube. Look at where the water line sits relative to the overflow tube.

If the water is at or above the overflow tube: Water is constantly flowing into the bowl via the overflow tube. You need to lower the float.

• Ball float (older toilets): Look for a small adjustment screw where the float arm connects to the fill valve. Turning it counterclockwise (or bending the arm slightly downward) lowers the float and drops the shut-off point.
• Cup float (modern fill valves): Look for a clip or pinch tab on the side of the vertical fill valve shaft. Pinch the clip, slide the float cup down the shaft, and release. Even half an inch lower can make a significant difference.

After adjusting, flush the toilet and let it refill. Check the new water level. Keep adjusting until the water stops about 1 inch below the overflow tube top.

If the float is cracked or waterlogged: On old ball float systems, the plastic ball can crack and fill with water, making it too heavy to rise properly and shut off the fill valve. In that case, the float needs to be unscrewed and replaced, or the entire fill valve assembly should be upgraded to a modern unit.

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Step 4: Replace the Fill Valve

If adjusting the float didn’t stop the running — or if you hear hissing, the fill valve cycles on and off by itself (ghost flushing), or water runs out of the fill valve itself — the fill valve needs to be replaced.

Modern fill valve replacements are inexpensive ($10–$20) and the install is simple. The Fluidmaster 400A is the most popular replacement on the market and fits virtually every toilet. Here’s how to replace it:

1. Turn off the water supply valve and flush the toilet to drain the tank. Use a sponge to absorb the remaining inch or so of water at the bottom of the tank.

2. Disconnect the water supply line where it connects to the bottom of the tank. Have a towel ready for residual drips.

3. Remove the old fill valve. Reach under the tank and locate the plastic lock nut on the fill valve shank (it protrudes through the bottom of the tank). Turn the nut counterclockwise to remove it. The old fill valve will lift out of the tank.

4. Install the new fill valve. Insert the new valve into the tank hole, adjusting the height per the instructions (most modern valves adjust by twisting). Tighten the lock nut by hand until snug, then add 1/4 turn with a wrench — don’t overtighten, as the tank can crack.

5. Reconnect the water supply line and turn the water back on. Let the tank fill. You’ll need to set the water level using the adjustment mechanism on the new valve. Most fill valves have a height adjuster or float clip. Set the water level 1 inch below the overflow tube.

6. Connect the refill tube. The small tube running from the fill valve to the overflow tube is the refill tube — it refills the bowl trap after a flush. Clip it to the top of the overflow tube. Make sure it doesn’t hang inside the tube, as that creates a siphon that causes running.

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Step 5: Check and Trim the Overflow Tube

If the fill valve is brand new or recently replaced and the toilet is still running, look carefully at the overflow tube height. Sometimes the tube itself is too tall for the tank, meaning the water can only rise to the correct level if the overflow tube is trimmed.

The overflow tube should be about 1 inch below the critical level (CL) mark on the fill valve — typically 1 inch below the tank lid’s underside or at the fill valve’s indicated mark. If it’s too tall, you can cut it down with a hacksaw. Make a clean, square cut and smooth the edge with a file. This is an unusual situation but does occur when a fill valve is replaced on an older toilet where the original overflow tube was calibrated for a different valve height.

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Other Causes of Running Toilets

Worn flush valve seat: If you’ve replaced the flapper and it still leaks (confirmed by the dye test), the problem may be the flush valve seat itself — the smooth plastic ring the flapper presses against. Mineral deposits, cracks, or roughness on the seat prevent a proper seal. You can sometimes fix this by cleaning the seat with fine steel wool. If it’s deeply cracked, the entire flush valve needs replacing, which is a more involved repair (requires removing the tank) but still DIY-able.

Loose chain or chain caught under flapper: If the flapper chain is too long, it can get tangled and hold the flapper slightly open. Check that the chain isn’t caught under the flapper after each flush. Trim the chain length if needed.

Cracked tank: Rare, but if you see water on the floor and the toilet is running, check the outside of the tank for hairline cracks. A cracked tank typically needs full replacement.

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How Much Does Fixing a Running Toilet Save?

The math here is compelling. A toilet with a moderate flapper leak wastes about 200 gallons per day. At a typical water rate of $0.005 per gallon, that’s $1 per day, or $365 per year — for a problem you can fix with a $7 flapper. A severe leak running 1,000 gallons per day wastes $1,825 per year. A toilet repair kit that fixes all the components at once costs $15–$25 and pays for itself in less than two weeks.

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Cost Breakdown

Repair	Parts Cost	Time Required
Replace flapper	$5–$15	10 minutes
Adjust float	$0	5 minutes
Replace fill valve	$10–$20	30 minutes
Full repair kit (all components)	$15–$25	45 minutes

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Related Reading

• How to Fix a Washing Machine That Won’t Drain
• How to Fix a Water Heater Pilot Light
• How to Unclog a Kitchen Sink Drain
• How to Fix a Leaky Faucet

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A running toilet is almost always a quick, cheap DIY repair. Start with the dye test to confirm whether the flapper is leaking, then work through the steps in order: replace the flapper, adjust the float, replace the fill valve. In most cases, you’ll solve the problem within the first two steps. The parts are inexpensive, the tools are minimal, and the water savings make this one of the highest-ROI home repairs you can make.