How to Fix a Running Toilet

A running toilet isn’t just annoying—it can quietly waste up to 200 gallons a day, enough to add real money to your water bill every month. The good news: 90% of the time it’s one of two cheap parts, and you can swap either in under 30 minutes with no plumber and no special skills.

First, find out why it’s running

A toilet “runs” when water keeps moving through the tank instead of sitting still. There are only two directions it can go: out the bottom into the bowl, or in the top past the overflow tube. A two-minute test tells you which.

Take off the tank lid and add a few drops of food coloring to the tank water. Wait 15 minutes without flushing. If color shows up in the bowl, water is leaking past the flapper at the bottom—that’s your culprit. If the bowl stays clear but you can see or hear water trickling into the overflow tube, the fill valve or float is set too high.

Tip: Before you buy anything, shut off the supply at the wall valve and flush to empty the tank. If your shut-off valve is frozen or weepy, add a new quarter-turn valve to your parts list while you’re at it—you’ll want it working before any repair.

The three parts that cause almost every running toilet

Inside every tank are just a handful of working components. Knowing what each one does makes diagnosis obvious.

Fix #1: Replace the flapper (the most common cause)

The flapper is the rubber or silicone seal at the bottom of the tank. Over a few years it hardens, warps, or gets a mineral film that keeps it from sealing—so water leaks into the bowl and the fill valve keeps topping the tank off. Flappers are cheap and universal-ish, but bring your old one (or a photo) to match the size; 2-inch is standard, but many newer high-efficiency toilets use a 3-inch flapper.

• Shut off the supply and flush to drain the tank.
• Unhook the old flapper from the overflow tube pegs and unclip the chain from the flush lever.
• Snap the new flapper onto the same pegs and reconnect the chain with about a half-inch of slack—too tight and it won’t seal, too loose and it won’t lift fully.
• Turn the water back on, let the tank fill, and run the food-coloring test again to confirm the leak is gone.

Fluidmaster and Korky make reliable, widely-compatible flappers you’ll find on any hardware store’s toilet-repair aisle. If the new flapper still leaks, run your finger around the flush-valve seat—a rough or pitted seat won’t seal against any flapper, and you’ll want a seat repair kit or a full flush valve.

Fix #2: Adjust or replace the fill valve

If your coloring test came back clear but water keeps running into the overflow tube, the tank is overfilling. First try the free fix: lower the float so the fill valve shuts off before water reaches the top of the overflow. On modern column-style valves (like a Fluidmaster 400) you pinch a clip and slide the float down; on older ball-and-arm valves you bend the arm down slightly.

Set the waterline about an inch below the top of the overflow tube. If the valve still hisses, drips, or won’t shut off after adjusting, the valve itself is worn—replace it. A universal fill valve is a $10–$20 part, installs with one locknut under the tank, and comes with clear instructions. You’ll want a pair of channel-lock pliers for the supply and locknut connections.

Tip: Doing one repair is a good moment to do both. If your toilet is 7+ years old, a complete repair kit (flapper + fill valve together, often sold as a Fluidmaster all-in-one) costs about the same as buying parts separately and resets the whole tank to like-new.

Don’t forget the supply side

While the tank is empty and your tools are out, glance at the parts that feed it. A crusty, corroded supply line or a shut-off valve that only half-closes will fail on its own schedule—usually at the worst time. A braided stainless supply line is a couple of dollars and far more burst-resistant than the old ribbed plastic ones. If the shut-off drips or won’t turn cleanly, swapping in a modern quarter-turn valve now saves you a soaked bathroom later.

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Frequently asked questions

A running toilet is one of the most satisfying repairs you can knock out in an afternoon—cheap parts, no special tools, and an immediate drop in your water bill. Bring your old flapper to your local independent hardware store to match it exactly, or ask us here if you’re not sure which part is causing the trouble.