How to Fix a Running Toilet (Step-by-Step Guide)
A running toilet wastes up to 200 gallons a day. Here is the actual 15-minute fix, flapper, fill valve, chain, using a $5 part instead of a $125 service call.
Quick answer: To fix a running toilet, lift the tank lid and check three things in order: the flapper (the rubber seal at the bottom, replace if water trickles past it, the cause about 80% of the time), the fill valve (adjust the float so water sits an inch below the overflow tube), and the chain (about half an inch of slack when the flapper is sealed). Most fixes cost under $20 and take 15 to 30 minutes. Hand-tighten the plastic nuts, cranking them with a wrench cracks the tank.
A running toilet wastes up to 200 gallons a day and sounds urgent. It almost never is. The flapper, a $5 rubber disc at the bottom of the tank, is the cause about 80% of the time. The other 20% is the fill valve or the chain, both of which take about as long to swap. A plumber’s minimum service charge runs $125 to $200; the parts cost less than a coffee.
Work through the three checks below in order.
Cost Math: Plumber vs. Yourself
Before we start, the numbers, because this is what makes the case for doing it yourself:
| DIY | Plumber | |
|---|---|---|
| Flapper ($5 part) | $5 | $125-200 minimum service call |
| Fill valve replacement | $12 | $150-250 |
| Chain | $2 | included in service call |
| Time | 15-30 min | wait for the appointment |
A plumber’s truck-rolling fee alone costs more than a new flapper at retail. You can pull the old one and walk into a hardware store with it in a paper towel in the time it takes a plumber to call you back.
What You’ll Need
Don’t pre-buy parts. Diagnose first, take the worn piece to the hardware store, and match it. “Universal” toilet parts usually mean “close enough to leak.”
- Rubber or latex gloves (optional, tank water is clean, but it’s still tank water)
- An adjustable wrench
- A sponge or old towel
- A new flapper ($5, match the one you pull out)
- A new fill valve ($12, Fluidmaster 400A is the one I default to; widely available, rated 7 to 10 years)
- A new flush chain ($2, if needed, usually comes with the fill valve)
Step 1: Turn Off the Water and Look Inside the Tank
Find the small valve behind the toilet near the floor and turn it clockwise to shut the water off. Not strictly required for diagnosis, but it stops the running while you look, and you’ll need it off anyway for the fix.
Lift the tank lid and set it carefully on the floor. Porcelain lids are heavier than they look and crack the second they hit tile, a folded towel underneath helps. Inside the tank you’ll see four things that matter:
- 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, tells it when to stop
- The chain, connects the flush handle to the flapper
Turn the water back on and flush a couple of times while you watch. Most running toilets give themselves away in under 30 seconds. You’re looking for: water trickling into the bowl, a fill valve that won’t shut up, or water spilling into the overflow tube.
Step 2: Diagnose Which Part Is Causing the Run
Three usual suspects.
Cause 1: The flapper isn’t sealing
After a flush, the flapper drops down and seals the hole at the bottom of the tank. If you can see a thin trickle of water running past it into the bowl, or if the tank water level drops over time without anyone flushing, the flapper is your problem. This is the cause roughly 80% of the time, start here.
Press down on the flapper with a finger. If the running stops while you hold pressure, that’s your answer. Korky and Fluidmaster both rate their flappers at around 5 to 7 years in normal water. In hard-water areas, or if you’ve been dropping blue chlorine tablets in the tank (don’t, see the FAQ), figure 1 to 2 years.
Cause 2: The fill valve won’t shut off
If you can hear the fill valve hissing long after the flush, it isn’t sensing that the tank is full. Either the float is set too high, or the valve itself is worn out. (For the opposite problem, a toilet that won’t fill up at all, the diagnostic is different, start with the supply valve at the wall.)
Look at the water level. It should sit about an inch below the top of the overflow tube, that’s the tall vertical pipe in the middle of the tank. If water is spilling INTO the overflow tube, the float is too high and the toilet will run forever; the tank physically cannot fill above that point.
Cause 3: The chain is wrong
If the chain between the flush handle and the flapper is too short, it pulls the flapper open a hair and water trickles out non-stop. If it’s too long, it gets pinned under the flapper as it tries to close. About half an inch of slack when the flapper is sealed is right, enough that a gentle tug on the handle lifts the flapper cleanly.
Step 3: Fix the Flapper
If the flapper is your problem:
- Turn off the water and flush to drain the tank.
- Sponge any remaining water out of the bottom.
- 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. Don’t force it, if it won’t move, it’s clipped, not glued.
- Take the old flapper to the hardware store and match it. Universal flappers fit most toilets, but exact-match flappers seal better and last longer. For a Kohler, get the Kohler one. For an American Standard, get the American Standard one. The model is stamped inside the tank lid.
- Slide the new flapper onto the posts, hook the chain up with about half an inch of slack, and turn the water back on.
- Flush and watch. If the tank fills cleanly and the running stops, you’re done. If you had trouble matching the right flapper or the seal still isn’t right, replacing a toilet flapper covers sizing, brand selection, and seat issues in more detail.
Step 4: Adjust or Replace the Fill Valve
Start with the easy fix, adjust the float.
Ball float (older style with a metal arm and round ball): Bend the arm gently downward. The bend lowers the ball, which tells the valve to stop filling sooner. Small movements; re-flush and check between.
Cup float (newer style that slides up and down the fill valve): There’s an adjustment screw or clip on top of the valve. Turn or slide it to lower the float.
You want a water level about an inch below the top of the overflow tube. Most tanks have a fill line stamped on the porcelain, use that as your target.
If adjusting doesn’t help and the valve is clearly toast (hissing, slow to fill, water spits out the top):
- Water off, flush, sponge the tank dry.
- Disconnect the water supply line under the tank with a wrench.
- Unscrew the plastic nut on the bottom of the tank that holds the fill valve in place.
- Lift the old fill valve straight out.
- Install the new one per the package, they’re height-adjustable for tank depth.
- Reconnect the supply line, water back on, set the float.
A new Fluidmaster 400A is $12 at any hardware store, versus $150 to $250 for a plumber to install one.
Step 5: Fix the Chain
Unhook the chain from the flush lever and re-hook it on a different link until there’s about half an inch of slack. Test by flushing, the flapper should lift cleanly and drop fully.
If the chain is rusty, kinked, or shedding flecks into the tank, replace it. They’re $2 and clip in by hand. Most fill-valve kits include a new one.
Where People Go Sideways
Cranking the plastic nuts. This is the one that turns a $5 fix into a $300 problem. The nuts that hold the fill valve and the supply line are plastic. They need hand-tight plus maybe a quarter turn with a wrench. Putting your shoulder into them will crack the tank, and a cracked tank means a new toilet.
Buying a “universal” flapper without checking. Universal flappers fit most toilets, but a lot of newer high-efficiency models, and most Kohlers, need a specific size. If the universal is in and it still trickles, check the brand and model stamped inside the tank lid and get the matching part. They cost the same.
Skipping the shutoff valve. Tempting to skip for a quick chain adjustment. Worth the two seconds it takes, bumping a fill valve mid-fix with the water on means a wet sock and a wet bathroom.
Setting the water level too low. Common overcorrection: after years of listening to a running toilet, people panic-lower the float until the tank barely fills, and then flushes don’t clear. Aim for that fill line stamped inside the tank. It exists for a reason.
Drop-in chlorine tablets. The blue or white tank-cleaning tablets eat the rubber inside the tank, flappers, fill valve seals, supply gaskets. Plumbers on /r/Plumbing say the same thing every time the question comes up: don’t. If you want bowl cleaning, drop the tablet in the bowl, not the tank.
Frequently Asked Questions
How much water does a running toilet waste? The EPA puts a continuously running toilet at up to 200 gallons a day, call it $20 to $70 extra on your monthly water bill depending on your local rates. Over a year, that pays for the entire toolset for most of the repairs on this site.
Why does my toilet run only sometimes? Almost always a flapper that’s slightly warped, seals most of the time but not always. Rubber doesn’t age evenly. Replace it; $5, fifteen minutes.
Is it bad to leave a running toilet for a few days? Mechanically, no, nothing’s going to break. Financially, the longer you wait the more expensive it gets. Pick a Saturday morning.
How long does a toilet flapper last? Korky and Fluidmaster both rate their flappers at 5 to 7 years in normal water. Hard water mineral buildup, chlorine tablets, and harsh tank cleaners cut that to 1 to 2 years. If you’ve replaced a flapper twice in five years, you probably have hard water, that’s a different fix.
Do I need to call a plumber? For a basic running toilet, no. A plumber comes in when 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 flush valve issue, which usually means swapping the whole inner assembly. Still a DIY job, but a longer Saturday.
My toilet runs AND gurgles. Same problem? No, two separate issues. The running is the tank parts (this article). The gurgle is a drain or vent restriction in the plumbing, see our toilet gurgling diagnostic for that side of the fix.
Four parts. Clean water. A flapper that costs less than your morning coffee. Start there, work down the list. When it finally stops, you’ll hear the quiet settle back in and wonder why you waited three days.