How to Fix a Toilet That Won’t Stop Running After Flush
The chain length, flapper, or flush valve seat — in that order — solve 95% of constant-running toilets.
TL;DR
A toilet that keeps running after flushing has one of three problems: chain too long (free fix — adjust), worn flapper ($8 part), or corroded flush valve seat ($0 — sand smooth). Try in that order. Total time: 15–30 minutes.
What’s in this guide
- Adjust the chain
- Replace the flapper
- Sand the flush valve seat
A toilet that keeps running after you flush — the water doesn’t shut off when the tank refills — is wasting up to 200 gallons of water per day. The fix is one of three simple things in 95% of cases. Here’s the order to try them.
Tools & Materials You’ll Need
Tools
- Adjustable wrench
- Sponge
- 400-grit wet/dry sandpaper
Materials
- Replacement flapper ($8) — if needed
Step-by-Step Instructions
Check the Chain Length
Lift the tank lid. The chain connects the flush handle to the flapper. Too long = chain prevents flapper from sealing. Adjust by hooking the chain link closer to the handle (less slack). Should have about 1/2 inch of slack when the flapper is closed.
Test the Flapper
If chain is right but toilet still runs, the flapper is leaking. Visual check: rubber should be smooth and intact. Push the flapper down firmly while watching — if water still flows, replace it. Replacement is $8 and 5 minutes.
Sand the Flush Valve Seat
If you’ve replaced the flapper twice and the toilet still runs, the seat (the rubber/plastic ring around the drain at the tank bottom) is corroded with mineral buildup. Drain tank, wipe the seat clean, sand lightly with 400-grit, apply thin smear of plumber’s silicone, reinstall flapper. Solves about 30% of repeat-flapper-failure cases.
When sanding the flush valve seat, use very light pressure with the wet sandpaper. You’re polishing, not removing material. 30 seconds of careful sanding is enough. Aggressive sanding can pit the seat and make the problem worse.
Don’t over-tighten flapper retainer
Some flappers attach with a small screw or clip. Snug is enough.
Real Scenarios from Our LA Service Calls
Three flappers in one year
Homeowner had replaced the same flapper three times in 12 months. Each replacement worked for a few weeks then failed. Sanded the flush valve seat at our visit. New flapper has been holding for 14+ months now.
When to Call a Plumber Instead
DIY isn’t always the right call. Bring in a licensed plumber if any of these apply:
- You’ve sanded the seat and replaced the flapper, still running
- Tank cracks visible
- Chain or fill valve broken in unusual ways
Frequently Asked Questions
My new flapper leaks immediately — defective?
Often it’s the wrong sub-model. Manufacturers make 5+ flapper variants per brand. Match to your specific toilet model number.
How much water does a constantly running toilet waste?
200+ gallons per day = $30+/month. Fix immediately.
Will softened water help my flapper last?
Yes. Soft water doesn’t deposit mineral buildup on the flapper or seat. Flappers in softened-water homes last 8+ years vs 4–5 in hard water.
Can I just replace the whole flush valve assembly?
Yes — comes with new flapper and seat together. $25–$40 and 60 minutes. Worth it if your tank has multiple issues.
Why does my toilet run only sometimes?
“Ghost flushing” — water level drops below the flapper threshold periodically, triggering refill. Same fix: tighten the seal.
Need professional help in Los Angeles?
Same-day service. Flat-fee pricing. No surprise add-ons.
Call (818) 938-8660
