Check the information indicated on the sidewall of the tyre (diameter and width). For example, 700x35 means that the tyre is 700mm in diameter and 35mm wide.
On the tyre sidewall you will also find the dimensions in mm in ETRTO international units.
For example, 36-622 means that the tyre is 35mm wide and has a 622mm internal diameter (or around 700x35).