The tank of my toilet runs like you just flushed every 15 minutes, why

Probably the flapper at the bottom of the tank. Sometimes you can replace just the flapper or may have to replace the whole mechanism inside the tank

Hi Margaret. We had the same problem. We just needed to tighten the chain that goes to the flapper.

Jody is right, it is the flapper.

You have two separate mechanisms in the tank. One is responsible for keeping the tank full. From your description that one is okay and doing its job.

The other is connected to the handle and opens a valve called a "flapper" when you push that handle. Your flapper is leaking and when water drops a few inches the fill valve opens and tops off the tank.

Replacement flapper can be had for less than $5. Turn the water off, flush to empty the tank, slip the old one off and disconnect from chain. New one on, reconnect chain, and turn the water on. Done. Easier than changing a light bulb in a ceiling fixture.

It needs the inner flapper replaced. Also, if you use a toilet cleaner in the tank, make sure it's not under the flapper. Flappers are not costly and easy to replace.

The flapper is good advise but also make sure that there is nothing ( cellophane, other stuff) trapped under the flapper causing it to stay open just enough for water to drain

Check to make sure there is some slack in the chain so the flapper falls and seals the flush valve opening. Make sure the water level is at least one inch below the overflow pipe on the flush valve (adjust the float). If all is good then replace the flapper.

In one bathroom, the chain from the arm to the bathroom would get twisted over time. You had to remove the chain from the arm, untwist it and then reattach it. In a different bathroom, I had to remove the flapper soak it in vinegar to get gunky deposits off of it and then put the flapper back in. My old house was from the 1800s so my fixtures were on the older side.