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A toilet that keeps refilling usually has water escaping from the tank or a fill mechanism that does not shut off properly. You can often narrow down the cause by watching one flush. The useful first step is observation, not turning every adjustment in the tank.

This guidance applies to a conventional gravity-flush toilet. Pressure-assisted, concealed, and unfamiliar systems need their own service instructions, and pressurized components should not be opened as an exploratory repair.

Make the inspection manageable

Remove the tank lid with both hands and set it on a padded, stable surface. Porcelain chips easily, and a replacement lid can be difficult to find. Keep tools from falling into the tank.

Locate the toilet's water shutoff before touching internal parts. Turn it only gently if you need to isolate the supply, and use the toilet's service procedure to confirm isolation before removing parts. Water already stored in the tank can still flush after the supply is closed; the valve's success cannot be judged only by whether a flush is possible. If the valve is seized, leaks, or fails to stop the water, do not force it; arrange a plumbing repair.

If water is leaking onto the floor, treat that as a separate problem requiring prompt attention. A running sound inside the tank is different from a leaking supply hose or cracked porcelain.

Watch the flush and refill

Flush once and observe the flapper or canister seal, the chain or linkage, and the rising water level. The flush mechanism should close completely, and the fill valve should stop at the level specified for the toilet.

A chain that is too tight can hold a flapper slightly open. Excess slack can catch beneath it. Adjust only according to the mechanism's instructions, leaving enough freedom for a full seal and normal handle movement.

If the handle sticks, check whether its linkage returns smoothly. Do not overtighten the handle assembly or bend parts without understanding their intended adjustment.

Distinguish a leaking seal from overfilling

Water flowing into the overflow tube suggests the fill level is too high or the fill valve is not shutting off. Use the marked waterline and the valve maker's adjustment procedure. Do not raise the level above the toilet's specification to improve flushing.

A tank that loses water and refills intermittently may have a leaking flush seal. A leak-detection dye permitted by the toilet maker and used as directed in the tank can help reveal water reaching the bowl without flushing; follow the product directions and rinse afterward to avoid staining.

Inspect the seal for debris, distortion, or deterioration. Use gentle cleaning methods approved for the parts. A worn flapper or canister seal should be replaced with a compatible part, not merely one that looks approximately the same.

Keep the refill arrangement correct

The small refill tube commonly sends water toward the overflow opening through a specified clip. Follow the toilet's instructions for its position and required air gap. Pushing it too far down the overflow tube can cause improper operation.

Do not alter the overflow height or bypass backflow-protection features. If the fill valve needs replacement, confirm compatibility, isolate the water successfully, and follow the installation directions closely. Corroded connections or uncertain access justify calling a plumber.

Avoid aggressive in-tank cleaning products unless the toilet manufacturer permits them. Some can damage seals and shorten component life.

Confirm that the repair holds

Restore water slowly and watch the complete refill. Check all disturbed connections with a dry tissue for moisture, then flush several times. The tank should refill to its intended level and remain quiet afterward.

Recheck after a period without use. Recurring refills, persistent leaks, or a valve that will not shut off deserve further service. Keep the water isolated if it is safe to do so while waiting for help, rather than leaving an unresolved fault running unattended.