Check the information indicated on the sidewall of the tyre (diameter and width). For example, 700 x 35 means your tyre is 700mm in diameter and 35mm wide.
On the sidewall, you will also find the dimensions in mm in international ETRTO measurements.
For example: 35-622 means your tyre is 35mm wide with an inner diameter of 700mm (so 700 x 35)