Look for the tyre size code on the sidewall
Read the first number as tyre width in millimetres
Read the second number as aspect ratio, the sidewall height as a percentage of width
Read the letter after the aspect ratio as tyre construction type, usually R for radial
Read the next number as rim diameter in inches
Read the load index number as maximum load rating
Read the speed rating letter as maximum speed capability
Check for extra markings such as XL, LT, or M+S if present
Match the full tyre size exactly when replacing tyres