How to Fix a Running Toilet (Step-by-Step Guide)
A running toilet wastes up to 200 gallons of water a day. Here's exactly how to diagnose and fix the three most common causes — flapper, fill valve, and chain — in under 30 minutes.
If your toilet won’t stop running, you’re in the right place. A running toilet is one of the most common plumbing problems homeowners face — and one of the easiest to fix yourself. In most cases, you can sort it out in 15 to 30 minutes with parts that cost less than $20 from any hardware store.
Before you call a plumber, work through the simple checks below. Nine times out of ten, the fix is something you can absolutely handle. We’ll go through exactly how to fix a running toilet, what tools you need, and the small mistakes that send people back to the store for a second trip.
What You’ll Need
You may not need all of these — work through the diagnosis first, then grab what’s required.
- A pair of rubber or latex gloves
- An adjustable wrench
- A sponge or old towel
- A new flapper (about $5 — bring the old one to the store to match it)
- A new fill valve (about $10–15, if needed)
- A new flush chain (about $2, if needed)
Don’t pre-buy anything until you’ve identified the problem. Toilet parts are not one-size-fits-all.
Step 1: Turn Off the Water and Look Inside the Tank
Before anything else, find the small valve behind the toilet near the floor and turn it clockwise to shut off the water. This isn’t strictly required, but it stops the running while you diagnose.
Lift the lid off the tank and set it carefully on the floor — porcelain lids crack easily. Take a look inside. You’ll see three main parts:
- The flapper — the rubber piece at the bottom that lifts when you flush
- The fill valve — the tall plastic tower on one side that refills the tank
- The float — connected to the fill valve, telling it when to stop refilling
- The chain — connects the flush handle to the flapper
Turn the water back on and flush the toilet a couple of times while watching the tank. Most running toilets give themselves away within 30 seconds of observation.
Step 2: Diagnose Which Part Is Causing the Run
There are three common causes. Here’s how to tell which one you have.
Cause 1: The flapper isn’t sealing
After you flush, the flapper should drop down and seal the hole at the bottom of the tank. If you can see a small trickle of water running past it into the bowl, or if the tank water level is dropping over time without anyone flushing, the flapper is the culprit. This is by far the most common cause.
Press down on the flapper with your finger (the water’s clean — but wear gloves if it bothers you). If the running stops while you press, you’ve found your problem.
Cause 2: The fill valve won’t shut off
If water keeps trickling into the tank long after a flush — and you can hear the fill valve hissing — the valve isn’t sensing that the tank is full. Either the float is set too high, or the valve itself is worn out.
Look at the water level. It should sit about an inch below the top of the overflow tube (the tall vertical pipe in the center of the tank). If the water is spilling into that overflow tube, the float is set too high and the toilet will run forever.
Cause 3: The chain is too tight (or too loose)
If the chain between the flush handle and the flapper is too short, it holds the flapper open slightly and water trickles out constantly. If it’s too long, it can get caught under the flapper when it tries to close.
There should be just a little slack — about half an inch — when the flapper is sealed.
Step 3: Fix the Flapper
If the flapper is your problem:
- Turn off the water and flush to drain the tank.
- Use a sponge to soak up any remaining water in the bottom of the tank.
- Unhook the chain from the flush lever.
- Slide the old flapper off the two posts on either side of the overflow tube. Some types twist off; some pop off with a gentle pull.
- Take the old flapper to the hardware store and match it. Universal flappers exist but exact matches seal better.
- Slide the new flapper into place, hook up the chain (with about half an inch of slack), and turn the water back on.
- Flush and watch. If the water rises and the tank stops filling cleanly, you’re done.
Step 4: Adjust or Replace the Fill Valve
If the fill valve is the issue, start with the easy fix: adjust the float.
For a ball float (the older style with a metal arm and a round ball): Bend the arm gently downward. This lowers the float and tells the valve to stop filling sooner.
For a cup float (the newer style that slides up and down the fill valve): There’s usually a small adjustment screw or clip on top. Turn or slide it to lower the float.
Aim for a water level about an inch below the top of the overflow tube.
If adjusting doesn’t help and the valve is clearly worn (hissing, slow to fill, or completely failed):
- Turn off the water, flush, and sponge out the tank.
- Disconnect the water supply line under the tank with a wrench.
- Unscrew the plastic nut on the bottom of the tank holding the fill valve in place.
- Lift the old fill valve out.
- Install the new one per the package instructions — they’re usually adjustable for tank depth.
- Reconnect the water supply, turn the water back on, and adjust the float to the proper level.
A new fill valve costs about $10–15 and takes 15 minutes.
Step 5: Fix the Chain
Easiest fix in plumbing. Unhook the chain from the flush lever and rehook it on a different link until there’s just a little slack — half an inch is right. Test by flushing. The flapper should lift cleanly and drop fully.
If your chain is rusty or kinked, replace it. They cost $2 and clip in by hand.
Common Mistakes to Avoid
Overtightening the plastic nuts. The nuts that hold the fill valve and supply line are plastic. They only need to be hand-tight plus a quarter turn with a wrench. If you crank down on them with full force, you’ll crack the tank or strip the threads. Then you have a much bigger problem.
Buying a “universal” flapper without checking your toilet model. Universal flappers fit most toilets, but some (especially newer high-efficiency models) need a specific size. If you’ve put in a universal and it still leaks, look up your toilet’s brand and model and get the matching one.
Forgetting to turn the water off. It’s tempting to skip this for a quick chain adjustment, but if you bump the fill valve mid-fix, you’ll get sprayed. Two seconds at the shut-off valve saves a wet bathroom.
Setting the water level too low. People sometimes overcorrect to stop running and end up with a weak flush. The water level should be about an inch below the top of the overflow tube — most tanks have a fill line marked on the inside.
Frequently Asked Questions
How much water does a running toilet waste? A continuously running toilet can waste 100 to 200 gallons of water per day. That’s an extra $20–70 on your water bill every month, depending on your local rates.
Why does my toilet run only sometimes? This is almost always the flapper. As rubber flappers age, they warp slightly, so they seal most of the time but not always. Replace the flapper — they’re cheap and the only “sometimes” fix.
Is it bad to leave a running toilet for a few days? Aside from the water waste, no — it won’t damage anything. But it’s not worth waiting. The fix is fast and cheap.
How long does a toilet flapper last? Most flappers last 4 to 5 years. Hard water, chlorine tablets, and harsh cleaners shorten that. If you drop chlorine tablets directly into your tank, expect to replace the flapper every year or two.
Do I need to call a plumber? For a basic running toilet, no — not unless you’ve replaced the flapper, fill valve, and chain and it’s still running. At that point, you may have a cracked overflow tube or a problem with the flush valve itself, which usually means replacing the whole inner assembly or, occasionally, the toilet.
You Got This
A running toilet looks intimidating when you lift the lid for the first time, but it’s actually one of the most beginner-friendly home repairs you can do. The parts are cheap, the tools are minimal, and the inside of a toilet tank — yes, the clean side — is a lot less mysterious than it looks. Start with the flapper, work your way through, and you’ll have it fixed before the kettle’s boiled.