The simple test
- Remove the toilet tank lid carefully and set it somewhere safe.
- Add several drops of food coloring—or a toilet leak-detection dye tablet—to the tank water.
- Do not flush.
- Wait roughly 10–30 minutes.
- Look at the water in the toilet bowl.
If colored water appears in the bowl without flushing, water is leaking from the tank into the bowl.
What usually deserves inspection
A worn or poorly sealing flapper is a common cause, but the flush valve seat, chain adjustment, fill level and other tank components can also contribute. Confirm the actual cause before replacing parts.
Why a silent leak matters
A toilet can lose water without leaving a puddle on the floor. Because the water travels directly from tank to bowl, the first clues may be periodic refilling, movement in the bowl or an unexpectedly high water bill.
Make it part of a home checkup
Test toilets periodically and whenever a toilet begins refilling on its own. Also inspect around the toilet base, supply line and shutoff valve for moisture; those are different leaks and require a visual inspection rather than this dye test.
Important: If water is leaking onto the floor, the shutoff valve does not work, the toilet is loose, plumbing is damaged, or you are uncertain about the repair, consult a qualified and licensed professional familiar with local codes and requirements.