Get the tablets
Toilet leak detection dye tablets
A pack costs a few dollars and tests every toilet in the house more than once — cheaper and faster than driving to a hardware store when you actually need it.
Check Price
How-To · Last updated August 1, 2026
No sound, no puddle, no obvious sign — just a water bill that's higher than it should be. Here's a free 10-minute test that catches it, using something you can find at almost any hardware store for a few dollars.
Every other leak on this site announces itself somehow — a stain, a puddle, a musty smell. A toilet leak often doesn't. The most common cause is a worn flapper valve, the rubber seal at the bottom of the tank that's supposed to hold water until you flush. Once it degrades, water seeps continuously from the tank into the bowl and down the drain — invisible, silent, and running 24 hours a day until someone notices the bill.
Beyond the worn flapper covered above, a few other common causes are worth ruling out. Hard water buildup can prevent the flapper from sealing fully even if it isn't old. The fill valve itself can fail and let water run continuously rather than shutting off at the right level. And the flush handle or the chain connecting it to the flapper can bend or come loose, holding the flapper open just enough to leak without anyone noticing the handle is the actual problem. Most of these show up in the same dye test below regardless of which one is the actual cause — the test tells you something's wrong before you need to know exactly why.
A silent flapper leak is the most common cause, but not the only one. If the tank refills periodically with no one flushing — listen for the fill valve cycling on its own — that's also a leak, just at a different point in the system. And if you've ruled out the tank entirely and still suspect water loss, the leak may be at the base of the toilet where it meets the floor, which won't show up in a dye test at all since it's not passing through the bowl. A wobbly toilet, a soft spot in the flooring around the base, or discoloration on the ceiling below (if there's a bathroom underneath) all point to a base leak instead — see our guide on finding a hidden water leak for how to track that down.
A worn flapper is one of the more DIY-friendly plumbing fixes in the house — a replacement flapper costs a few dollars and takes about ten minutes to swap, no tools usually required. A leak at the base, though, means pulling the toilet to replace the wax ring seal, which is a bigger job and where it's reasonable to call a plumber if you're not comfortable doing it yourself.
Get the tablets
A pack costs a few dollars and tests every toilet in the house more than once — cheaper and faster than driving to a hardware store when you actually need it.
Check Price
FAQ
Yes — a worn flapper lets water seep from tank to bowl constantly, with no puddle or sound, just a higher water bill.
Anywhere from tens to hundreds of gallons a day depending on severity — real money on a metered bill over a few weeks.
Reliably, and it's a simple test — color in the bowl without flushing means water's getting past the flapper.
Check the base of the toilet instead — a base leak won't show up in a dye test. See our hidden leak guide for that.