Most running toilets come from a worn flapper, mis-set fill valve, or a refill tube causing overflow; quick tests pinpoint the part.
If the tank refills every few minutes or you hear a steady hiss, you likely have a leak from the tank to the bowl or a fill valve that will not shut off. A small leak can dump 200 gallons in a day, which adds up fast. You can read the same warning from a large utility and from the EPA WaterSense program, which urges quick checks during Fix a Leak Week and year-round.
Why a toilet keeps running: main culprits
Most running toilets trace back to one of a few parts: the flapper or seal, the fill valve and float, the refill tube in the overflow, the chain and handle, or the flush valve seat. The table below links everyday symptoms to the likely cause and a quick no-tool test.
| Symptom you notice | Likely cause | Quick test |
|---|---|---|
| Steady hiss after a flush | Fill valve not closing; water level set too high | Lift the float by hand; if the sound stops, tune or replace the fill valve |
| Water trickles into bowl | Worn flapper or seal; rough flush valve seat | Do a dye tablet test in the tank; color in the bowl means a leak past the flapper |
| Tank refills every few minutes (ghost flush) | Slow leak past flapper or flush valve | Mark tank water line with a pencil; if it drops without a flush, the seal leaks |
| Water level rides into overflow tube | Float set too high; refill tube inserted too far | Lower the float and clip the refill tube above the overflow opening |
| Handle feels sticky or chain snags | Chain too tight or kinked; handle arm binding | Leave 1/2 inch slack in the chain; make sure the arm moves freely |
| New toilet runs after shutoffs or work | Sediment stuck in fill valve cap | Flush the fill valve per maker steps to clear grit |
How a toilet tank works in plain terms
When you press the handle, a lever lifts the chain and opens the flapper (or a canister seal). Water leaves the tank through the flush valve into the bowl. The drop in level lowers the float and opens the fill valve. The fill valve sends water into the tank and a small stream through the refill tube into the overflow to top off the bowl. When the float reaches the set line, the fill valve should shut. If any part sticks, leaks, or is set wrong, the water never rests.
Two water levels matter. The tank level should rest on the mark cast in the porcelain or sit a finger below the top of the overflow. If either level drifts, the tank will cycle. Getting these lines right fixes many “never stops” complaints without any new parts.
Causes of a constantly running toilet: DIY fixes
Worn or warped flapper or seal
Rubber hardens with age and with chlorine exposure. A flapper that no longer seals will let water seep into the bowl until the tank drops and refills again. Do a fast dye test: add a dye tablet or a few drops of food color to the tank and wait 15–20 minutes without flushing. Color in the bowl means the flapper leaks. If your model uses a canister seal, the same test applies.
Swap the flapper correctly
Match the part to the valve type and flush volume. Some toilets need a specific flapper or seal kit to meet the designed flush. Many brands list the exact part number on the inside of the tank or in the manual. Shut the supply, drain the tank, unhook the old flapper, clean the seat, and install the new one with the chain set so the flapper closes freely. Test twice to confirm a clean seal.
Fill valve out of tune or dirty
If the float sits too high, water rides into the overflow tube and never stops. Set the water line to the mark inside the tank or about an inch below the top of the overflow. If the valve hisses or cycles, grit may be stuck in the cap. Many valves can be flushed clean in place. Open the cap per the maker’s steps, hold a cup over the opening, and crack the supply for a few seconds to purge debris. If the valve still misbehaves, swap in a new anti-siphon fill valve and set the float.
Refill tube siphoning
The small tube that tops off the bowl should clip to the overflow and aim into the opening without going down inside. If the tube sits below the rim, it can siphon water and draw the level down, which restarts the fill. Clip the tube so its tip rests above the rim and use the anti-siphon clip if supplied. This tiny tweak ends a lot of mystery refills.
Chain and handle problems
A chain set too tight can hold the flapper open. A chain set too loose can snag. Leave a small loop so the flapper can seat cleanly. Make sure the handle arm moves up and down without rubbing the tank lid or the side of the tank. Metal arms can bend slightly to clear the lid. If the handle sticks, clean mineral buildup on the pivot nut and retighten by hand.
Flush valve seat damage
A pitted or cracked seat lets water slip by even with a new flapper. Run a finger around the seat to feel for rough spots. Minor pitting can sometimes be smoothed with a repair kit made for that brand. If the seat or the overflow tube is cracked, replace the flush valve. That job means removing the tank, so pick a new seal kit and new tank bolts while you are in there.
Sediment after supply work
Any work that turns water on and off can shake loose grit that lands in the fill valve. If your toilet starts to run right after supply work, clear the cap on the valve and flush it per the maker steps. If the valve still will not seal, install a new one.
Simple tests that save time
Dye test: Color in the bowl without a flush points to the flapper or the flush valve seat.
Pencil line test: Draw a line at the water level, wait, and check for drop. A falling line means the tank is losing water.
Float lift test: With the tank open and water running, gently lift the float. If the sound stops, the fill valve was open; tune or replace it.
Refill tube test: Pull the tube up so its tip sits above the overflow rim and clip it. If the cycle stops, you had a siphon.
Water waste, bills, and why speed matters
Even a small leak can burn through a lot of clean water in one day. A large city water office warns that an open fill valve can dump thousands of gallons in a single day, and many utilities report that a small leak can hit 200 gallons per day. Quick checks with dye or a pencil mark can stop that loss. The NYC DEP leak page explains simple tests and encourages fast repairs so bills stay under control. If your water meter moves with all fixtures off, a toilet leak is a top suspect and worth checking first.
Parts, tools, and setup
You do not need a shop full of tools. A new flapper or a fill valve, an adjustable wrench, a sponge or towel, and a bucket handle most jobs. Shut off the supply at the stop valve, flush and hold the handle to drain the tank, and mop up the last inch with a sponge so your hands stay dry while you swap parts.
Lay parts out on a towel and take a quick photo of the tank before you start. That snapshot makes it easy to match hose routing and chain length later. Do not overtighten plastic parts; snug by hand, then a small nudge with a wrench is enough.
| Fix or check | Typical time | Typical part cost |
|---|---|---|
| Replace flapper or canister seal | 10–20 minutes | 8–20 USD |
| Adjust float and water line | 5–10 minutes | 0 USD |
| Flush or replace fill valve | 15–30 minutes | 12–35 USD |
| Clip refill tube above rim | 2–5 minutes | 0–2 USD |
| Replace flush valve and tank bolts | 45–90 minutes | 15–40 USD |
Step-by-step: replace a flapper
1) Shut off the supply. 2) Hold the handle to drain the tank. 3) Unhook the chain and the side ears of the old flapper. 4) Clean the seat with a cloth. 5) Hook the new flapper and connect the chain with slight slack. 6) Turn the water on and test. If the leak remains, the seat may be the problem.
Step-by-step: tune or swap a fill valve
1) Set the water line to the mark in the tank. 2) If the valve hisses, lift the float; if the sound stops, the valve is open. 3) Shut off the supply, open the top per the maker guide, and flush out grit. 4) If the cycle still runs, install a new anti-siphon fill valve and set the float so the water line rests below the overflow by about an inch. 5) Clip the refill tube so its tip stays above the rim.
Step-by-step: stop a siphon from the refill tube
1) Take off the tank lid. 2) Find the small tube that runs from the fill valve to the overflow. 3) If it sits inside the overflow, pull it up so the tip rests above the rim. 4) Use the clip that came with the valve so it stays put. 5) Test a few flushes to confirm the bowl refills and the tank stops cleanly.
When to call a pro
Call for help when you see cracks in the tank, water on the floor, rusted tank bolts that will not move, a stuck supply stop, or if the overflow never drops even with the supply closed. If several toilets in the home run at once, you may have pressure or supply issues that need a licensed plumber to sort out.
Prevent repeat trouble
Use quality parts that match your model. Keep dye tabs on hand for fast tests. Note the flush volume stamped on the bowl and tank so you buy parts that fit that rate. If your toilet predates current high-efficiency models, a WaterSense labeled unit can cut use per flush and still clear the bowl. See the EPA page on residential toilets for the label and what it means. During Fix a Leak Week and any time you hear a hiss or see ripples in the bowl, run a dye test and a quick float check.
Avoid drop-in tank tablets that claim to clean with each flush. Those bricks can break down rubber and shorten the life of flappers and seals. Wipe the tank parts with a soft cloth during your next swap and clear any grit from the cap of the fill valve. A few minutes of care keeps parts moving smoothly and the tank quiet.
Quick checklist
✓ Water level rides into overflow? Lower the float.
✓ Color leaks into bowl? Replace flapper or seal.
✓ Hiss stops when you lift the float? Clean or replace the fill valve.
✓ Refill tube sits inside overflow? Clip it above the rim.
✓ Chain snags or holds flapper open? Reset the slack.
✓ Seat rough or cracked? Plan on a flush valve kit.