Every tire carries its size on the sidewall in a code such as 225/45R17 94W. Once you know what each part means, you can read any tire at a glance.
The first number is the section width in millimetres, measured from sidewall to sidewall. In 225/45R17, the tire is 225 mm wide.
The second number is the aspect ratio: the sidewall height as a percentage of the width. A 45 means the sidewall is 45% of 225 mm, so a little over 100 mm tall. Lower numbers mean a shorter sidewall and usually sharper steering with a firmer ride.
The letter that follows is the construction. R stands for radial, which is what almost every modern car tire uses.
The next number is the rim diameter in inches - a 17 means the tire fits a 17-inch wheel. Get this wrong and the tire simply will not mount.
The final pair is the load index and speed rating. 94W means the tire can carry 670 kg and is rated to 168 mph. Never fit a tire rated below what your vehicle's placard asks for.
Not sure what your car takes? Save your vehicle in My Cars and we will only show you sizes that fit.