Use this simple test to determine if your toilet flapper is leaking and needs to be replaced.
Your toilet gets a lot of use, so normal wear-and-tear results in a lot of maintenance. One of the biggest bathroom issues is a toilet that keeps running after you flush it or runs by itself without being flushed. If left untreated, this problem can run up your water bill, so you should fix this problem as soon as possible. A new toilet nay be required, but test it first. There may be a simpler fix.
These problems are typically rooted in the fill tank. The first issue could be the water level. If the water level is above the overflow pipe, then the fill valve or ballcock isn't working properly or is set to the wrong height. Make sure that the water level is adjusted to the manufacturer's instructions. It's usually stamped inside the tank.
If the internal diaphragm seal is worn, replace it. Some fill valves cannot be required and the entire fill valve must be replaced.
Be sure to check the flapper. If the flapper is leaking, it will keep triggering the fill valve to refill the tank. To see if the flapper is leaking, here is a simple test:
Before you replace it, first check the flapper to see if sediment or debris has built up around it. If not, replace it. This is typically a lot cheaper and easier to replace than the fill valve.