Why Is My Toilet Running? 6 Common Causes (and Fixes)
That faint hissing sound coming from the bathroom at 2 a.m. isn't your imagination — a running toilet can waste anywhere from a few gallons to hundreds of gallons a day, depending on the cause. The good news is that most of the culprits are simple, inexpensive parts you can find at any hardware store. Here are the six most common reasons, roughly in order of how often we see them.
1. A worn-out flapper
The flapper is the rubber (or plastic) piece at the bottom of the tank that seals the drain hole between flushes. Over time it warps, cracks, or develops a mineral coating that keeps it from sealing fully. This is the single most common reason for a running toilet, and it's a five-minute fix: shut off the water, unhook the old flapper, and snap in a new one.
2. A chain that's too long or too short
The little chain connecting the flush lever to the flapper needs just enough slack to let the flapper seat fully — too much slack and it can get caught under the flapper, too little and it holds the flapper open a crack. Adjust the chain length so there's about a half-inch of slack.
3. A fill valve that won't shut off
If you hear water running continuously (not just after a flush), the fill valve itself might be failing to shut off once the tank reaches the right level. This is usually a sign the valve is simply worn out and needs replacing — a slightly more involved job than a flapper swap, but still within reach for most DIYers.
4. The float is set too high
If your fill valve has an adjustable float, and it's set too high, water keeps filling past the shutoff point until it spills into the overflow tube — which just looks like a toilet that never stops running. Adjusting the float down slightly often solves it instantly.
5. A cracked overflow tube
Take the tank lid off and watch what happens after a flush. If water is visibly pouring into the overflow tube even after the tank should be full, the tube itself may be cracked, or the fill valve is simply set too high (see above). A cracked tube needs to be replaced rather than adjusted.
6. Mineral buildup on the flush valve seat
In homes with hard water, mineral deposits can build up on the valve seat the flapper sits on, preventing a clean seal even with a brand-new flapper. A gentle scrub with a soft brush (tank water off and drained) can sometimes resolve it; heavy buildup may mean the whole flush valve assembly needs attention.
When to stop troubleshooting and call a plumber
If you've replaced the flapper, checked the chain, and adjusted the float and the toilet is still running — or if you notice water on the bathroom floor, a toilet that rocks, or the problem coming back within days of a fix — there may be an issue with the tank-to-bowl seal or the fill valve itself that's best diagnosed by a professional. It's also worth calling in help if you're not comfortable shutting off and draining the tank safely.
A running toilet rarely fixes itself, and every day it runs is a few more dollars on your water bill. Most of the time it's a quick DIY win — but if the fixes above don't stick, a licensed plumber can usually solve it in one visit.