What does your tyre size mean?
Every tyre has a string of numbers and letters stamped on the sidewall, such as 205/55 R16 91V. It tells you the width, profile, wheel size, how much weight the tyre can carry and how fast it's rated to go. Use this free tyre size calculator and decoder to translate yours into plain English, then find the right tyres for your car or browse tyres by manufacturer.
On the sidewall
The full size is moulded into the outer rubber of every tyre.
Six key parts
Width, profile, construction, rim, load index and speed rating.
Match your car
Use the exact size your manufacturer recommends for safe fitment.
Where to find your tyre size
Look at the outer sidewall of any tyre on your car. You'll see something like this stamped into the rubber:
205/55 R16 91V
↑ This is your tyre size
Decode your tyre size
Enter your size exactly as it appears on the sidewall, then press Decode.
Try one of these popular sizes
How to read your tyre size
Tyre markings look complicated, but every car tyre size follows the same pattern. Take 205/55 R16 91V as an example - reading left to right, here is what each part of the tyre size means:
| Part of the size | Example | What it means |
|---|
| Section width |
205 |
The width of the tyre across the tread, measured in millimetres. |
| Profile (aspect ratio) |
55 |
Sidewall height as a percentage of the width - here, 55% of 205 mm (about 113 mm). |
| Construction |
R |
Radial construction, the standard for modern car tyres. |
| Rim diameter |
16 |
The wheel diameter the tyre fits, measured in inches. |
| Load index |
91 |
The maximum weight each tyre can carry - 91 equals 615 kg. |
| Speed rating |
V |
The maximum speed the tyre is rated for - V equals 149 mph (240 km/h). |
Enter your own tyre size in the decoder above to see this breakdown for your car. Once you know your size you can compare tyres from leading brands that fit it, then book fitting near you.
Your tyre dimensions explained
Car tyre sizes are written in metric format, and every number describes a real tyre dimension. The first number is the section width (also called the tyre width or tread width), the distance across the tread in millimetres. The second number is the profile, or aspect ratio, which sets the sidewall height as a percentage of the width: a taller sidewall gives more comfort, a lower profile sharper handling.
The letter R means radial, and the last number is the rim diameter in inches - the wheel diameter, or rim size, the tyre fits. From these figures this tyre calculator works out the overall diameter (the full tyre diameter, written as tire diameter in the US), the rolling circumference and the revolutions per mile.
These tyre dimensions matter whether you drive a small passenger car or a light truck, because they decide how the tyre fits the wheel, how accurately it reads on the speedometer and how the car handles.
Popular tyre sizes
Jump straight to some of the UK's most-searched tyre sizes:
Changing tyre size and plus sizing
If you are fitting bigger alloy wheels or weighing up alternative tyre sizes, the aim of plus sizing is to increase the wheel size while keeping the overall diameter close to your original tyre size. Run a tyre size comparison to check any new tyre size against the old one: as long as the difference in diameters stays within about 3%, the fitment and gearing stay safe.
A larger difference in diameters changes how far the tyre travels per turn, so the speedometer and odometer no longer read true. A bigger overall diameter causes speedometer error, where your actual speed is higher than the dial shows; a smaller diameter does the opposite. A tyre size comparison calculator (also written tire size comparison calculator) compares the two sizes and shows the speedometer error so you know your actual speed before you buy.
Tyre size FAQs
What does my tyre size mean?
Your tyre size is the string of numbers and letters on the sidewall, such as 205/55 R16 91V. Reading left to right it gives the section width in millimetres, the profile (aspect ratio), the construction type, the rim diameter in inches, the load index and the speed rating - together they define which tyres fit your car.
What is the tyre profile or aspect ratio?
The profile, or aspect ratio, is the sidewall height as a percentage of the tyre's width. On a 205/55 tyre the sidewall is 55% of 205 mm - about 113 mm. A lower profile sharpens handling, while a higher profile gives a more comfortable ride.
What does the tyre load index mean?
The load index is a code for the maximum weight one tyre can carry at its rated pressure. A load index of 91, for example, equals 615 kg per tyre. Always fit tyres that meet or exceed the load index your vehicle requires.
What is the tyre speed rating?
The speed rating is a letter showing the maximum speed a tyre is approved for. V is rated to 149 mph (240 km/h), W to 168 mph (270 km/h) and Y to 186 mph (300 km/h). Always match or exceed your car's recommended speed rating.
Can I change my tyre size?
You can sometimes fit an alternative tyre size, but keep the overall rolling diameter within about 3% of the original so your speedometer stays accurate and the tyres don't rub. When in doubt, use the size recommended by your vehicle manufacturer.
Where do I find my tyre size?
Your tyre size is moulded into the outer sidewall of every tyre. You'll also find the recommended sizes on a sticker in the driver's door shut, inside the fuel filler flap, or in your vehicle handbook.
Looking for more?
Check tyre pressures, tread depth, speed ratings and more with our free tyre tools.
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