How can I tell whether a toilet tank uses a bottom-entry or side-entry fill valve?

To identify the correct type of toilet fill valve, first check the water inlet hole position on the toilet tank. If the inlet hole is located at the bottom of the tank, the toilet uses a bottom-entry fill valve. If the inlet hole is located on the side wall of the tank, it uses a side-entry fill valve.

Before replacing or purchasing a fill valve, it is recommended to confirm the inlet position and inlet thread size. This helps ensure the new fill valve fits the tank properly and avoids installation problems.