This guide covers diagnosing and repairing a running toilet — a toilet that continues to trickle or run after the flush cycle is complete, or one that periodically refills the tank without being flushed. A running toilet wastes between 200 and 1,000 gallons of water per day depending on severity, making it one of the highest-impact plumbing repairs a homeowner can make. The fix is almost always one of three components: the flapper, the fill valve, or the float. Identifying which one is the problem takes five minutes. The repair itself rarely exceeds one hour and costs between $7 and $35 in parts.
This guide covers the standard two-piece gravity-flush toilet with a separate tank and bowl, which accounts for roughly 85% of residential toilets in North America. Pressure-assist toilets, wall-hung toilets, and one-piece toilets with concealed tanks have different internal mechanisms and are noted where the approach differs.
Time: 30–90 minutes. Cost: $7–$35. Difficulty: Beginner. Permit required: No. Tools required: Adjustable pliers, sponge, bucket, towels. Parts: Flapper (most common), fill valve kit, or float assembly depending on diagnosis.
The Three-Part Diagnosis
Before buying any parts, identify which of the three components is causing the run. Each has a simple test.
Diagnosis A — The Dye Test (flapper leak)
Drop three to five drops of food coloring into the toilet tank. Do not flush. Wait ten minutes. If color appears in the bowl without flushing, the flapper is not sealing and is allowing water to leak continuously from tank to bowl. This is the most common cause of a running toilet, accounting for approximately 70% of cases. The flapper is the rubber gasket seated at the bottom of the tank that opens to allow the flush and closes to seal the tank while it refills.
Diagnosis B — The Float Test (high water level)
Remove the tank lid and observe the water level while the toilet is not running. The water level should be approximately 1 inch below the top of the overflow tube (the vertical standpipe in the center of the tank). If the water level is at or above the top of the overflow tube, water is continuously spilling into the bowl via the overflow — not via the flapper. This means either the float is set too high, or the fill valve is not shutting off when it should. Mark the current water level on the inside of the tank with a pencil.
Diagnosis C — The Flush Handle Test (chain issue)
Observe the chain connecting the flush handle arm to the flapper. If the chain has too much slack, it can catch under the flapper and prevent a full seal. If the chain is too tight, it holds the flapper partially open. The correct chain length leaves approximately 1/2 inch of slack when the flapper is fully seated.
What You Will Need
Tools
Adjustable pliers (channel-lock style, 10-inch)
Sponge and small bucket
Old towels for floor protection
Flashlight (for inspecting inside the tank)
Food coloring (for dye test)
Measuring tape or ruler
Parts — buy after diagnosis
Flapper replacement — Universal flapper (Fluidmaster 502P or equivalent), $5–$10. Or bring the old flapper to the hardware store for an exact match by brand (Kohler, American Standard, TOTO, etc.).
Fill valve replacement — Fluidmaster 400A fill valve or equivalent universal replacement, $12–$18. Required if the fill valve body is cracked, the diaphragm is worn, or the valve does not shut off at any float height adjustment.
Float ball — Only for older toilets with a ball float on a horizontal arm. $6–$10.
Complete rebuild kit — Fluidmaster 400CRP or equivalent, $20–$35. Includes flapper, fill valve, and flush valve seat repair. Recommended if the toilet is over 10 years old or if multiple components are suspect.
Step-by-Step Repair
Step 01 · Shut off the water supply valve
The water supply valve is the oval or football-shaped valve on the wall behind and below the toilet, on the left side when facing the toilet. Turn it clockwise until it stops — typically 3–5 full turns. Flush the toilet once to empty the tank. If the valve is old, stiff, or corroded and difficult to turn, do not force it with a wrench — the packing nut can crack or the valve body can shear on older galvanized valves. Call a plumber if the shut-off valve is non-functional. See the bathroom repair guide index for more on supply valve replacement.
Step 02 · Remove the tank lid and sponge out remaining water
Lift the tank lid straight up — it is not attached, just resting on the tank. Set it flat on a towel away from the work area. Use the sponge and bucket to remove the remaining 1–2 inches of water from the bottom of the tank. A dry tank makes the repair easier and prevents spills on the floor when disconnecting components.
Step 03 · Replace the flapper (if Diagnosis A confirmed)
Unhook the flapper ears from the pegs on either side of the flush valve seat (the ring at the bottom of the tank). Disconnect the chain from the flush handle arm. Take the old flapper to the hardware store if you are unsure of the brand, or buy a universal replacement. Attach the new flapper ears to the flush valve pegs, reconnect the chain with 1/2 inch of slack, and confirm the flapper seats fully flat over the drain opening. Turn the water supply valve back on. Flush twice and perform the dye test again to confirm the seal. Total repair time: 10–15 minutes.
Step 04 · Adjust or replace the fill valve (if Diagnosis B confirmed)
If the water level is running into the overflow tube, the fix is either to lower the float setting or replace the fill valve entirely. On modern float cup fill valves (the tall cylindrical valve with the float sliding up and down on the side), the water level is adjusted by turning the adjustment screw at the top of the valve clockwise to lower the water level, counterclockwise to raise it. The target level is 1 inch below the top of the overflow tube — this measurement is often marked as a line on the inside tank wall or on the valve body itself. Make adjustments in quarter-turns. Flush and allow to refill. Repeat until the water stops 1 inch below the overflow.
If adjusting the float setting does not stop the overflow, or if the fill valve continues to run after the water reaches the correct level, the fill valve diaphragm is worn and the entire valve should be replaced. To replace: disconnect the refill tube (the small flexible tube running from the fill valve to the overflow pipe), use channel-lock pliers to unscrew the lock nut at the bottom of the tank connecting the fill valve to the water supply line, lift out the old valve, and install the new Fluidmaster 400A or equivalent per its instructions. The valve height is adjustable to fit tanks from 7 to 13 inches deep. Reconnect the refill tube, turn on the water, and set the water level as above.
Step 05 · Adjust the float on older ball-float valves
Older toilets (pre-1990s) may use a ball float — a large hollow plastic or metal ball on the end of a horizontal arm connected to the fill valve. If the ball float is set too high, the valve never shuts off. Bend the float arm gently downward (toward the bottom of the tank) to lower the shutoff point. Modern replacement is the better long-term fix: a Fluidmaster 400A replaces ball-float mechanisms directly and eliminates the arm-bending adjustment entirely. If the float ball itself is cracked or waterlogged (it sinks rather than floats), it must be unscrewed from the arm and replaced with a new float ball.
Step 06 · Fix a chain length problem (if Diagnosis C confirmed)
Disconnect the chain from the flush handle arm. Count the links from the flapper clip. Reattach the chain at a link that gives 1/2 inch of slack when the flapper is fully closed. There should be no kinking, twisting, or debris caught under the chain path. If the chain is so long that it doubles back and gets caught under the flapper, cut the excess chain with pliers, leaving 2–3 extra links beyond the attachment point. A chain too short prevents the flapper from fully seating; a chain too long repeatedly catches under the flapper and holds it open.
Step 07 · Inspect the flush valve seat
If the flapper is new and seating correctly but the dye test still shows leakage, the problem is the flush valve seat — the rubber or ceramic ring the flapper seals against. Run a fingertip around the seat rim. Any nick, crack, groove, or mineral deposit on the seat allows water to bypass even a perfect flapper. Minor mineral deposits can be removed by scrubbing with a fine abrasive pad (Scotch-Brite or equivalent) while the tank is empty. Significant physical damage to the seat requires either a flush valve seat repair kit (a rubberized ring that adheres over the damaged seat, $8–$12) or full flush valve replacement, which requires draining and removing the tank from the bowl. Full flush valve replacement is a two-hour repair and is covered in the bathroom repair index.
Step 08 · Restore water and test
Turn the supply valve counterclockwise to fully open. Allow the tank to fill completely — the fill valve should shut off automatically when the water reaches the correct level. Flush the toilet and observe the full flush cycle: the flapper should lift fully, the bowl should clear completely, and the tank should refill and shut off cleanly with no continued running. Perform the dye test one final time and wait ten minutes. No color in the bowl confirms the repair is complete. Check the floor around the base of the toilet and under the tank for any drips from disturbed supply connections.
Common Mistakes to Avoid
Buying a flapper without checking the brand. Universal flappers fit most toilets, but TOTO and some Kohler models use proprietary seat geometry. A mismatched flapper leaks immediately or within a few weeks as the rubber conforms to an imprecise seat. Bring the old flapper for comparison, or note the toilet brand and model number from the inside of the tank lid.
Overtightening the fill valve lock nut. The lock nut at the bottom of the tank is plastic. Hand-tight plus one quarter-turn with pliers is sufficient. Over-tightening cracks the nut or the tank base — a far more expensive problem than a running toilet.
Setting the water level too low. The tank water level affects flush performance. Setting it more than 1.5 inches below the overflow tube results in weak flushes that require double-flushing — which wastes more water than the original slow leak.
Ignoring a stiff or non-functional shut-off valve. Do not attempt any toilet repair if the shut-off valve is frozen open and cannot be closed. The repair will become a flood. Address the valve first.
Replacing only the flapper when the seat is damaged. A new flapper on a grooved or pitted seat leaks within days. Always inspect and clean the seat when replacing the flapper.
Not testing with the dye test after repair. A toilet that sounds quiet can still have a slow leak that the dye test catches. Silent leaks from small flapper gaps waste 30–100 gallons per day without the audible running sound.
When to Call a Professional
A plumber is warranted when the shut-off valve behind the toilet is stuck, corroded, or leaking (replacing a supply valve requires turning off water at the main and may involve sweating copper); when the toilet rocks on the floor (indicates a failed wax ring seal at the floor flange, a separate repair requiring the toilet to be reset); or when water appears around the base of the toilet during or after flushing (also a wax ring failure). The internal tank repairs described in this guide are straightforward homeowner work. Work outside the tank — at the floor, the supply line, or the drain — typically warrants professional involvement.
Follow-Up Maintenance
Perform the dye test annually on every toilet in the house — many slow leaks develop silently between audible running events. Replace flappers proactively every 3–5 years if the toilet is heavily used; flappers cost $6–$10 and degrade from chlorine in municipal water over time. If the home has hard water (above 7 grains per gallon hardness), mineral buildup on the flush valve seat is a recurring issue — clean with white vinegar and a fine abrasive pad every 12–18 months. Fill valves last 5–10 years in typical use; include fill valve replacement in any bathroom renovation or toilet reset. For related bathroom repairs, see the bathroom repair index.
Filed by HowTo: Home Edition. This is a Repair × Bathroom guide. A running toilet is the highest-volume silent water waste problem in residential plumbing — a 200-gallon-per-day leak adds roughly $60–$180 annually to a typical water bill. The three-step diagnosis above resolves the cause in five minutes, and the fix costs under $35 in almost every case.
Common Questions About Running Toilet Repair
My toilet runs for a few seconds every hour without being flushed. What is happening? This is called "phantom flushing" or "ghost flushing." The tank water level is slowly dropping through a flapper that is not fully sealed, and when the level drops enough to trigger the fill valve's float, the fill valve opens briefly to top up the tank. Because the flapper is only partially open, the drip rate is slow and the refill cycle runs every 30–90 minutes rather than continuously. The dye test will confirm it: color in the bowl means the flapper is the cause. Replace the flapper as described in Step 03. Phantom flushing can waste 100–500 gallons per day depending on the rate of the slow leak.
I replaced the flapper and the toilet still runs. What else could it be? After a flapper replacement, the remaining causes in order of likelihood are: (1) the flush valve seat is damaged — run a fingertip around the seat ring and feel for any groove, nick, or mineral scale. A seat repair ring ($8–$12) adheres over the damaged seat surface. (2) The new flapper is the wrong type for your toilet — TOTO and Kohler use brand-specific seat geometry. Bring the old flapper to compare. (3) The fill valve is not shutting off and water is overflowing through the overflow tube — this is a separate problem from the flapper, diagnosed by whether the water level is at or above the overflow tube rim.
How do I know what brand my toilet is? The brand and model number are typically stamped on the inside of the tank lid (on the underside of the porcelain cover) or on the inside wall of the tank near the water line. Note both the brand and the model number — "Kohler" is not enough; "Kohler Wellworth K-3422" identifies the exact flush valve geometry.
Is it safe to use toilet tank tablets or drop-in cleaners? Chlorine-based tank tablets (the blue or green tablets that dissolve in the tank) accelerate the degradation of rubber flappers and fill valve diaphragms. A toilet treated with tank tablets typically needs flapper replacement every 12–18 months rather than every 3–5 years. If tank cleanliness is a priority, use a bowl cleaner applied to the bowl rather than a tank tablet that bathes the flapper in dilute bleach 24 hours per day.
What is the water level supposed to be in the tank? The correct water level is approximately 1 inch below the top of the overflow tube. Most fill valves have a water level line marked on the valve body. Setting the level lower reduces the flush volume and can cause weak flushes; setting it higher than the overflow tube top causes continuous running. The WaterSense standard for low-flow toilets (1.28 gpf) requires the correct water level to achieve complete bowl clearing — do not lower the level below the manufacturer's specification to save water, as double-flushing uses more water than the original level.
How much water does a running toilet waste? It depends on the failure mode. A slow flapper leak (ghost flushing): 100–500 gallons per day. A flapper that is open continuously (constant run audible as a hissing sound): 200–700 gallons per day. Water spilling continuously over the overflow tube (fill valve not shutting off): 400–1,000 gallons per day. At a typical residential water rate of $0.006 per gallon, a 500-gallon-per-day leak costs approximately $90 per month, or over $1,000 per year. A $10 flapper replacement is among the highest ROI maintenance items in a home.
Water and Cost Impact Reference
Flapper replacement: $6–$10, resolves 70% of running toilet cases.
Fill valve replacement (Fluidmaster 400A): $12–$18, resolves most remaining cases.
Complete rebuild kit (Fluidmaster 400CRP): $20–$35. Recommended for toilets over 10 years old — replaces flapper, fill valve, and flush handle in one kit.
Professional plumber: $75–$150 service call for a running toilet repair, parts additional. DIY saves $65–$140 for a repair that takes under one hour.
Water waste at 500 gal/day: approximately $1,095/year at national average water rates. The $10 flapper pays for itself within hours of installation.
Understanding Toilet Fill Valve Types
Three generations of fill valve are still in active use in residential toilets, and the adjustment procedure differs for each. Knowing which type is installed determines whether the repair is a screw-turn, an arm-bend, or a full valve replacement.
Float cup fill valve (modern standard, post-1994). The tall cylindrical valve with the float riding on the side of the valve body on a plastic clip. The float rises and falls with the water level. Adjustment: turn the screw at the top of the valve clockwise to lower the water level setpoint, counterclockwise to raise it. Fluidmaster 400A is the most common example. This type adjusts easily and reliably. If the valve still runs after adjustment to the correct water level, the diaphragm inside the valve is worn — replace the valve.
Ball-float fill valve (older standard, pre-1994). A horizontal arm extending from the fill valve with a hollow ball at the end. As the water rises, the ball floats up and eventually closes the valve. Adjustment: bend the float arm downward to lower the shutoff point (close the valve earlier), or upward to raise it. The arm can be bent by hand or with pliers on older brass arms. Alternatively, a large thumb screw or knurled nut adjusts the arm length on some models. This type is less reliable over time — the float ball can become waterlogged and sink rather than float, requiring float ball replacement, or the arm pivot point corrodes. Replacement with a modern float cup valve (Fluidmaster 400A mounts in the same location) is a reliable long-term upgrade.
Diaphragm fill valve (less common, found in some Kohler and British-style toilets). A wide, flat valve body where the fill rate is controlled by a rubber diaphragm rather than a float directly. These valves are quiet and durable but require brand-specific repair parts. Adjustment is by a screw on top of the valve cap. If the diaphragm deteriorates, the valve either runs continuously or fails to refill — diaphragm kits are available for major brands at $8–$15.
For any fill valve that has been in service for over 10 years and is causing problems, replacement with a new Fluidmaster 400A or equivalent is the simplest and most reliable resolution — the cost difference between a repair diaphragm kit and a full new valve is under $5, and the new valve comes with a 5-year warranty.
Time: 30–90 minCost: $7–$35Difficulty: BeginnerBy: HowTo: Home Edition
A running toilet wastes 200–1,000 gallons of water per day. The cause is almost always one of three components — the flapper, the fill valve, or the float. Identify which before buying parts.
Dye test first: drop food coloring in the tank and wait 10 minutes without flushing. Color in the bowl = flapper. Water level above the overflow tube = fill valve or float. Chain caught under the flapper = chain length.
The Three-Part Diagnosis
Symptom
Cause
Fix
Food dye appears in bowl after 10 min
Flapper not sealing
Replace flapper ($7–$10)
Water at or above overflow tube rim
Float set too high / fill valve worn
Adjust float or replace fill valve ($12–$18)
Chain caught under flapper
Chain too long or slack
Adjust chain to ½ inch slack
New flapper still leaks
Flush valve seat damaged
Clean seat or install seat repair ring
What You Will Need
Adjustable pliers (channel-lock, 10-inch)
Sponge and small bucket
Food coloring for dye test
Replacement flapper — Fluidmaster 502P or brand-matched equivalent
Fill valve — Fluidmaster 400A if needed
Complete rebuild kit — Fluidmaster 400CRP for toilets over 10 years old
The Repair Steps
Step 01
Shut off the supply valve and empty the tank
Turn the oval valve on the wall behind the toilet clockwise until fully closed. Flush once to empty the tank. Sponge out the remaining 1–2 inches of water. If the shut-off valve is frozen or leaking, stop and address that first — do not attempt tank work with no way to stop the water.
Step 02
Replace the flapper
Unhook the flapper ears from the flush valve pegs. Disconnect the chain. Take the old flapper to the hardware store for an exact match if the brand is unclear. Attach the new flapper, reconnect the chain with ½ inch of slack. Turn on water, flush twice, run the dye test again.
Step 03
Adjust the fill valve float height
On float cup fill valves (the standard modern type), turn the adjustment screw clockwise to lower the water level. Target: 1 inch below the top of the overflow tube. Flush and allow to refill. Repeat until the valve shuts off cleanly at the correct level.
Step 04
Replace the fill valve if adjustment does not work
Disconnect the refill tube. Unscrew the lock nut at the tank bottom with channel-lock pliers — hand tight plus one quarter-turn only, the nut is plastic. Install Fluidmaster 400A per instructions. Reconnect refill tube and supply line. Turn on water and set the level.
Step 05
Inspect and clean the flush valve seat
Run a fingertip around the seat rim. Any nick, groove, or mineral scale allows water past a perfect flapper. Clean with a fine abrasive pad while the tank is dry. Significant damage requires a seat repair ring or full flush valve replacement.
Step 06
Restore water and run the final dye test
Turn on the supply valve, allow the tank to fill, and run the dye test for 10 minutes. No color in the bowl confirms the repair. Inspect the floor around the toilet for any drips from disturbed supply connections.
Common Mistakes
Buying a universal flapper for a TOTO or Kohler — these use proprietary seat geometry
Overtightening the fill valve lock nut — hand tight plus ¼ turn is enough
Setting the water level too low — weak flushes waste more water than the original leak
Replacing the flapper without inspecting the seat for grooves or mineral scale
