Why Your Toilet Keeps Running — Causes & Easy Fixes

Why does my toilet keep running and how do I make it stop?

A toilet that won't stop running almost always comes down to a worn flapper, and sometimes a misadjusted float or a worn fill valve — most are cheap, sub-30-minute fixes you can do yourself. Stopping the leak can save thousands of gallons of water a month.

ℹ️ Reference only: For general reference only. This guide does not guarantee any result — every home is different. Verify against your local building codes and a licensed professional before acting, especially for electrical, gas, plumbing, structural, or roof work.

💵 DIY parts: flapper $5-12, fill valve $10-20, complete tank repair kit $20-35. Plumber visit: $100-250 for a typical flapper or fill-valve job, $150-350+ if the flush valve or tank work is involved. ⏱ 10-20 minutes for a flapper or float adjustment; 20-40 minutes to replace a fill valve. ● Use caution

Safety: Low risk, but always shut off the supply valve before disconnecting anything to avoid a flood, and never overtighten plastic lock-nuts or the supply line — cross-threading or cranking too hard can crack the tank or fitting. If the shut-off valve won't close, shut off the home's main water before proceeding. Tank water is clean, but wear gloves if working in the bowl. No gas or electrical work is involved.

Common causes

Worn, warped, or dirty flapper not sealing the flush valve, so water leaks from the tank into the bowl and the fill valve keeps topping it off (most common)
Quick check: Put a few drops of food coloring in the tank, wait 15 minutes without flushing; if color shows up in the bowl, the flapper is leaking.
Water level (float) set too high, so water spills over the top of the overflow tube and drains continuously into the bowl (common)
Quick check: Lift the tank lid and look: if water is trickling into the top of the overflow tube (the open vertical pipe), the level is too high.
Flapper chain too short or tangled, holding the flapper slightly open after each flush (common)
Quick check: Check the chain between the flush lever and flapper; if it's taut or caught under the flapper, it can't seal.
Fill valve worn out or stuck, so it never fully shuts off (often a hissing or trickling sound) (common)
Quick check: If the dye test stays clean but you still hear water, lift the float up by hand; if the running stops, the fill valve isn't shutting off properly (worn valve or level set too high).
Mineral buildup or a cracked/calcified flush valve seat that prevents a good seal (less common)
Quick check: Run a finger around the flush valve seat (where the flapper rests); grit, pitting, or scale means it won't seal even with a new flapper.
Overflow tube cracked or too tall/short for the tank, letting water escape (less common)
Quick check: Inspect the overflow tube for cracks; water level should sit about 1 inch below its top rim.

How to fix it

Turn off the water at the shut-off valve under the toilet (turn clockwise), then flush to empty the tank before working.
Do the food-coloring (dye) test first to confirm whether the leak is at the flapper (color reaches the bowl) or elsewhere (no color but still running).
Fix #1 — Flapper: Unhook the old flapper from the overflow tube pegs and chain, take it to the hardware store to match it, and snap the new one on. Universal flappers fit most toilets; brand-matched ones (Korky, Fluidmaster) seal best.
Fix #2 — Chain: Adjust so there's about 1/2 inch of slack when the flapper is closed. Too tight holds it open; too loose can tangle. Trim excess chain so it can't get caught.
Fix #3 — Water level/float: Lower the water so it sits about 1 inch below the top of the overflow tube. On a modern column-style fill valve, pinch the clip and slide the float down; on an old ball-float, turn the adjustment screw on top counterclockwise or gently bend the float arm down.
Fix #4 — Fill valve: If it still hisses or won't shut off, replace the whole fill valve. A universal Fluidmaster 400A is about $12-15 and installs in 15-20 minutes — shut off water, disconnect the supply line, unscrew the lock-nut under the tank, swap it in, and reconnect hand-tight plus a slight turn (don't overtighten plastic nuts).
Fix #5 — Flush valve seat: If a new flapper still leaks, clean the seat with a sponge and white vinegar to remove scale. If it's pitted or cracked, install a flush-valve seal repair kit, or replace the flush valve (this requires removing the tank — see DIY vs Pro).
Turn the water back on slowly, let the tank refill, and re-run the dye test to confirm the leak is gone.

DIY or call a pro?

Flapper, chain, float, and fill-valve swaps are all confidently DIY — no special skill, just a sponge and maybe a wrench. Call a licensed plumber if a full flush-valve replacement means pulling the tank and the bolts are corroded, if the tank or bowl is cracked, if the shut-off valve itself leaks or won't turn, or if you've replaced the flapper and fill valve and it still runs (could be a hidden hairline crack or a bad supply line).

Tools & parts

Replacement flapper (universal or brand-matched)
Fill valve (e.g., Fluidmaster 400A) if needed
Food coloring or dye tablet for leak test
Adjustable wrench or channel-lock pliers
Sponge and towels
White vinegar (for cleaning the valve seat)
Bucket (to catch residual tank water)

Keep a record of every fix you make — what broke, what it cost, how you solved it.

Track your home's fixes in Home Story →

Based on: Manufacturer guidance (Fluidmaster, Korky flapper and fill-valve instructions); Reputable DIY references (This Old House, Family Handyman); EPA WaterSense guidance on leaky toilets and water waste; General plumbing-trade best practices

This is general home-maintenance guidance, not professional plumbing advice. Toilet designs vary by brand and age; always confirm against your specific fixture and any manufacturer instructions. If you're unsure, the shut-off won't close, or water damage is a risk, contact a licensed plumber.