How to Fix a Running Toilet

Updated August 2026 · Home Project Resources

A running toilet is almost always one of three parts: the flapper, the fill valve, or the float. A fifteen minute dye test tells you which one, and the part itself is one of the cheapest things any hardware store sells.

Drop food coloring in the tank, wait without flushing, and look in the bowl. Color in the bowl means the flapper is leaking. No color in the bowl but the tank still refilling means the fill valve or the float. That single test replaces all the guessing, and it's why this repair takes an hour instead of three trips to the store.

Rebuilding the whole bathroom instead? Our tile calculator sizes floor and wall tile in seconds, and the bathroom remodel cost guide lays out what fixtures, labor, and finishes actually run.

Updated August 2026

A note on numbers. This page gives you dimensions, quantities and scope, not dollar figures. Material and contractor pricing varies by region, by season and by the specifics of your job, and we do not set any of it. Work out what you need here, then get quotes that can be compared to each other.

What "running" actually means, and why it costs you

Two different symptoms get called a running toilet.

Constant running. You hear water moving all the time, or the fill valve never shuts off. Water is escaping the tank as fast as it comes in.

Phantom flush. The toilet is silent for ten or twenty minutes, then the fill valve kicks on for fifteen seconds and goes quiet again. The tank is leaking slowly into the bowl until the float drops enough to trigger a refill. Most people ignore this one for months.

Both are the same underlying fault in most cases, and both waste real water. A slow flapper leak that triggers a refill every fifteen minutes typically moves somewhere between 200 and 1,000 gallons a day. A hard running toilet, where the fill valve simply never shuts off, can push past 3,000 gallons a day.

Framing it in gallons rather than dollars is deliberate, because your water rate is local and your bill structure may be flat, tiered, or metered on a delay. What is not local: 200 gallons a day is more water than a family of four uses for everything else combined. If you're on a well, the pump is running for all of it. If you're on a septic system, every one of those gallons goes into the drain field, which is a vastly more expensive problem than a new flapper.

The dye test, described precisely

This takes two minutes of work and fifteen minutes of waiting. Do it before you buy any part.

1. Take the tank lid off and set it flat on a towel on the floor. Not balanced on the seat or the edge of the tub. Tank lids are porcelain, heavy, and no longer manufactured for most models.
2. Do not flush. The tank needs to be sitting at its normal full, settled level.
3. Add color to the tank water. Ten to fifteen drops of dark food coloring, or a toilet dye tablet if you have one. Blue, red, or green, anything but yellow. Swirl it gently with your hand or a spoon so it distributes.
4. Leave the toilet completely alone for 15 to 20 minutes. No flushing, and tell everyone in the house. Longer is fine, up to about 30 minutes. Beyond that, the dye can stain an older bowl.
5. Look in the bowl without flushing. You're looking for tint in the bowl water.

Here's what each result means.

Look at the overflow tube while you're in there. It's the vertical open pipe in the middle of the tank. Water should sit roughly an inch below its top rim. If the water level is at the rim or spilling into it, you have found your problem and you don't even need the dye test result to act on it.

Shut off and drain before any part swap

Every repair below starts here. It takes three minutes and it's what keeps a pocket-change job from becoming a wet floor.

1. Close the supply stop. It's the small valve on the wall or floor behind the toilet, where the flexible supply line attaches. Turn the oval handle clockwise until it stops, or turn a quarter turn lever so it sits perpendicular to the line.
2. If the stop won't turn, is seized, or weeps when you move it, stop and close the main water shutoff for the house. Old multi turn stops snap. The main is usually near the meter, where the line enters the house, or by the water heater. Forcing a corroded stop is how people flood a bathroom.
3. Flush and hold the handle down so the tank empties as fully as it will.
4. Sponge out the last inch. A sponge and a bucket, or a wet/dry vac. There's always a residual inch under the flapper that the flush won't take.
5. Put an old towel on the floor around the base. Assume you'll spill.
6. Disconnect the supply line at the tank only if you're replacing the fill valve. For a flapper you can leave it alone.

When you're done, open the stop slowly, about a quarter turn at a time, and watch the connections for the first full fill cycle.

Flapper: the cause about 80 percent of the time

The flapper is the rubber or silicone cup at the bottom of the tank that lifts when you flush and drops to seal the flush valve opening. It's the cheapest part in the tank and the one that fails first. Three separate things go wrong with it.

Warping and hardening. Rubber flappers stiffen and distort with age and with chlorine exposure, especially if you use in tank cleaning tablets. A flapper that no longer sits flat can't seal. Squeeze it: a good one is soft and springy, a failed one feels stiff, slick, or chalky, and often leaves black residue on your fingers.

Mineral buildup on the seat. The flush valve seat, meaning the plastic or brass ring the flapper lands on, collects scale in hard water areas. A perfect flapper on a rough seat still leaks. Run a finger around the seat: it should feel glassy smooth. Grit, ridges, or a sandy texture means the seat needs cleaning, and sometimes the whole flush valve needs replacing.

Chain length, which is the most common cause of all and the cheapest fix. This is worth checking before you buy anything.

• Too long and the slack chain gets pinned under the flapper as it drops, holding the seal open a hair. This produces a classic phantom flush.
• Too short and the chain keeps constant tension on the flapper, lifting it slightly. This produces a constant run.
• The sweet spot is about a half inch of slack with the flapper seated and the handle at rest.

Adjust by moving the clip to a different chain link, then cut off the excess with wire cutters or bend it back on itself. Leave three or four spare links, no more, because a long dangling tail is exactly what gets sucked under the flapper.

Replacing a flapper, step by step

1. Shut off and drain the tank, as above.
2. Photograph the existing setup before you touch it, including how the chain routes to the handle arm.
3. Unhook the chain from the handle arm.
4. Slide the flapper ears off the two pegs on the sides of the overflow tube. Some flappers use a ring that slips down over the overflow tube instead, so lift it off.
5. Take the old flapper with you to the store. Flappers come in 2 inch and 3 inch sizes and they are not interchangeable. A 3 inch flush valve is common on newer high efficiency toilets. Universal flappers cover most 2 inch valves, but matching the actual part is more reliable.
6. Clean the flush valve seat with a green scrub pad or fine steel wool and white vinegar. Scale comes off with patience. Do not use a metal blade or coarse sandpaper, because a scratched seat leaks forever.
7. Install the new flapper, seat it, and set the chain slack to about a half inch.
8. Turn the water on, let it fill, and flush twice. Watch the flapper close. It should drop cleanly and stay down.
9. Run the dye test again to confirm. Fifteen minutes now beats discovering it next month.

One flapper, 20 to 30 minutes.

One more flapper note. If your toilet was made before about 1994 and has a 3.5 gallon or larger tank, replacement flappers can be genuinely hard to find. Look for the model number stamped inside the tank wall or under the lid, and search that. Some old two piece toilets need a full flush valve replacement instead, which means pulling the tank off the bowl.

Fill valve: how to tell it's the culprit

The fill valve is the tall assembly on the left side of the tank that refills it after a flush. Newer ones are plastic float cup types. Older ones are the brass ballcock with a rod and a floating ball.

Signs it's the fill valve rather than the flapper:

• The dye test came back clean but the toilet still cycles or hisses.
• A constant hiss or whistle even when the tank is full.
• The fill never shuts off completely, so a thin stream keeps entering the tank.
• Water trickles from the top of the valve or from around its cap.
• The refill takes forever, over about 90 seconds, which points at a partly clogged valve or a partly closed supply stop.

Adjusting versus replacing

Try adjusting first, it's free. On a modern float cup valve there's a threaded rod or a spring clip on the side. Pinch the clip and slide the float cup down to lower the water level, or turn the adjustment screw counterclockwise. On an older ballcock, bend the brass rod gently downward, or turn the screw on top of the valve. Target a water level about one inch below the top of the overflow tube. Many tanks have a molded fill line or a stamped mark, and if yours does, use that.

Clean it before replacing it. Debris from municipal work or a water heater flush lodges in the valve seal and stops it sealing. Shut the water off, drain the tank, twist the cap off the top of the fill valve (usually a quarter turn), lift out the seal, hold an upturned cup over the open valve, and briefly open the supply to blow the grit clear. Reassemble. This fixes a surprising number of "failed" valves for nothing at all.

Replace it when the body is cracked, the cap won't come off, it hisses after cleaning, it's a brass ballcock older than about 20 years, or you've already tried adjustment and cleaning. A universal fill valve is inexpensive and fits nearly every toilet made in the last 40 years. Budget 30 to 45 minutes.

Replacement outline: shut off and drain, disconnect the supply line at the tank, hold the valve body inside the tank while you unscrew the plastic locknut underneath, pull the old valve out, set the new one to your tank height per its instructions before installing, seat the cone washer, hand tighten the locknut plus a quarter turn only, reconnect the supply, clip the small refill tube into the overflow tube (not down inside it, which can siphon), then open the water and check for drips.

Float and water level: reading the overflow tube

The float, whether it's a cup riding on the fill valve shaft or a ball on an arm, tells the fill valve when to stop. If it's set too high, the tank keeps filling past the top of the overflow tube, and the overflow tube dumps that water straight into the bowl.

This is the single most misdiagnosed running toilet. People replace a perfectly good flapper because they can hear water running, when in fact the water is pouring down the overflow tube and out of sight.

How to check it in ten seconds: take the lid off and look at where the water sits relative to the top of the overflow tube. Water should be about one inch below the rim, never at it, never over it.

If water is running into the overflow tube:

1. Lower the float using the adjustment described above.
2. Flush and let it refill completely, then look again. Repeat until the level settles about an inch low.
3. If the level keeps climbing past your setting, the fill valve is not shutting off and needs cleaning or replacing. The float adjustment is only the control, and a failed valve ignores it.
4. Check the small refill tube while you're there. It should clip to the top of the overflow tube and spray in from above. If someone pushed it down inside the tube, the tank can siphon into the bowl continuously.

One catch: don't lower the level so far that the flush gets weak. If you set the water two or three inches below the overflow and the bowl now needs two flushes, you've traded one water waste problem for a worse one. Bring it back up to an inch below and solve the real leak instead.

How to fix a running toilet: the diagnostic table

Every part in that column is a small purchase, and the whole list together is less than one plumber's visit. If you do hire it out, the number is set by the minimum service call in your market rather than by the part, so ask for the minimum and the hourly rate on the phone before booking, and ask whether the truck stocks common flappers and fill valves so it happens in one trip.

Worked example: a phantom flushing toilet, start to finish

A 2011 toilet that fires up its fill valve for about 12 seconds every 20 minutes, day and night.

• Minutes 0 to 2. Lid off, set on a towel. Water sits about an inch below the overflow tube rim, so the float is fine and the overflow tube is dry. That rules out the float immediately.
• Minutes 2 to 4. Twelve drops of blue food coloring in the tank, swirled.
• Minutes 4 to 22. Left alone.
• Minute 22. Clear blue tint in the bowl. Verdict: flapper side.
• Minutes 22 to 25. Looked at the chain. About two inches of slack, with the tail resting right across the flapper. Shortened it to a half inch of slack and cut the excess.
• Minutes 25 to 45. Flushed clean, ran the dye test again. Still tinted, though less. So the chain was part of it but not all of it.
• Minutes 45 to 75. Shut the supply stop, flushed, sponged the last inch out. Old flapper came off the pegs and felt stiff with a chalky surface. Seat had a gritty ring of scale, scrubbed off with a green pad and white vinegar over about ten minutes. New 2 inch flapper on, chain set to a half inch.
• Minutes 75 to 95. Water back on, two test flushes, third dye test. Bowl stayed clear.

Total: about 95 minutes including waiting, one flapper. At an estimated 250 gallons a day of leakage, that fix stopped roughly 7,500 gallons a month.

When to replace the whole toilet instead

Repair is the right call most of the time. These are the cases where it isn't.

• The tank or bowl is cracked. Hairline cracks spread. A cracked tank can fail all at once and dump several gallons on the floor.
• The flush valve seat is scored or corroded and a new flapper still leaks after descaling. Replacing a flush valve means pulling the tank, and at that labor level a new toilet is often the better spend.
• Parts are genuinely unavailable. Some 1980s and early 1990s models have proprietary tank hardware nobody stocks.
• It's a pre 1994 water hog. Toilets from before the 1.6 gallon standard use 3.5 to 7 gallons per flush. If yours runs 3.5 gallons and the household flushes 20 times a day, replacing it with a 1.28 gallon model saves roughly 16,000 gallons a year. That pays for the toilet.
• It clogs constantly even with a healthy flapper and a correct water level. Older low flow designs from the first generation of 1.6 gallon toilets had genuinely poor bowl geometry.
• You've replaced two or more tank components in the last year. At that point a full rebuild kit or a new toilet beats a third trip.

Replacing a toilet is two numbers, the fixture and the labor for a straight swap, and the fixture range is wide because a basic two-piece and a wall-hung or bidet-seat model are nothing alike. If you're doing it yourself, our toilet installation guide covers the rough-in measurement, flange height, and wax ring, which are the three things that decide whether the new one leaks.

Frequently asked questions

How much water does a running toilet actually waste?

A slow flapper leak that triggers a refill every 15 to 20 minutes commonly wastes 200 to 1,000 gallons a day. A hard running toilet where the fill valve never shuts off can exceed 3,000 gallons a day. To gauge yours, close the supply stop overnight and mark the tank level, or read your water meter before bed and again in the morning with nothing else running.

Why does my toilet run for a few seconds every 20 minutes?

That's a phantom flush, and it means the tank is leaking slowly into the bowl until the float drops far enough to trigger a refill. The cause is almost always the flapper or the chain. Check the chain slack first, since it's free, then run the dye test and replace the flapper if color reaches the bowl.

Can I fix a running toilet without turning the water off?

You can adjust a chain or a float with the water on. Any part swap, meaning a flapper, a fill valve, or a flush valve, needs the supply stop closed and the tank drained first. Trying to change a flapper in a full tank means working blind in cold water while the fill valve keeps refilling around your hands.

Do toilet cleaning tablets damage the tank parts?

In tank chlorine tablets shorten the life of rubber flappers, seals, and gaskets significantly, and many manufacturers void the warranty if you use them. If you want a bowl cleaner, use one that goes in the bowl, not the tank. This is a common reason a flapper that should last five years fails in eighteen months.

What size flapper do I need?

Measure the opening at the bottom of the tank, or measure the flapper you removed. Two inch is the long standing residential standard. Three inch flush valves appear on many high efficiency toilets made since roughly 2005 and give a faster flush. They are not interchangeable, so take the old part to the store or check the model number stamped inside the tank.

My toilet still runs after I replaced the flapper. What now?

Three usual causes. The flush valve seat is scaled or scored, so run your finger around it and descale with vinegar and a green pad. The chain is too short and holding the new flapper up. Or the real fault was never the flapper, and water is going over the overflow tube because the float is set too high. Look at the water level relative to the overflow rim.

Is a running toilet an emergency?

Not a flood emergency, since the water goes down the drain rather than onto the floor. It is urgent for your bill, for a well pump that's now cycling constantly, and especially for a septic system, where continuous flow can saturate the drain field. Close the supply stop between uses if you can't fix it today.

Should I just buy a full rebuild kit?

If the toilet is over about eight years old and more than one component is suspect, yes. A complete kit with fill valve, flapper, and flush valve gasket costs a little more than the two parts bought separately, takes about an hour, and resets the clock on every wear part in the tank. For a single failed part on a newer toilet, replace just the part.

The short version

Look at the overflow tube first, because it costs nothing and immediately rules the float in or out. Then run the dye test. Color in the bowl means the flapper side, so check the chain before you buy anything. No color plus a cycling valve means the fill valve or the float.

Shut the supply stop and drain the tank before you swap any part, keep the water level about an inch below the overflow rim, and re-run the dye test to confirm the fix. Nearly every running toilet is one small part and an hour.

If the toilet itself is done rather than just leaking, the toilet installation guide covers the replacement, and the bathroom remodel cost guide puts it in the context of the rest of the room. A dripping sink in the same bathroom is a different repair, covered in how to fix a leaky faucet.