Tyre Size Calculator

Use the TreadTorque Tyre Size Calculator to turn the numbers on a tyre sidewall into useful information.

Enter or select a tyre size, such as 205/55 R16 91V, to calculate its sidewall height, overall diameter, rolling circumference and other key dimensions. The calculator helps you understand the physical measurements behind a tyre size and what each part of the size designation means.

Tyre size calculator: frequently asked questions

A tyre’s size code contains several pieces of technical information. Understanding those numbers and letters can help you identify the dimensions of a tyre and make sense of the specifications shown on its sidewall. Use the tyre size calculator above to explore the information associated with the size you enter.

How do I read a tyre size?

A common passenger-car tyre size is 205/55 R16. The first number, 205, is the nominal section width in millimetres. The second number, 55, is the aspect ratio. It describes the sidewall height as a percentage of the tyre’s nominal width. In this example, the sidewall height is approximately 55% of 205 mm.

The letter R indicates radial construction, which is used by most modern passenger and light vehicle tyres. The final number, 16, is the diameter of the wheel rim in inches. The tyre size must correspond to the rim diameter it is designed to fit.

Some tyre sizes include additional information, such as a load index, speed rating, or markings for a particular vehicle application. For example, a size may be written as 205/55 R16 91V. The numbers and letters after the size describe other tyre specifications. They are separate from the tyre’s width, profile and rim diameter.

Is the rim diameter the same as the overall tyre diameter?

No. The rim diameter is the diameter of the wheel the tyre fits, shown in inches as the final number in a size such as 205/55 R16. The overall tyre diameter includes the rim plus the height of the tyre’s sidewall above and below it. That is why the complete tyre is larger than the rim alone.

Does the calculator tell me which tyre size to fit?

No. The calculator provides technical information based on the size entered. It does not verify that the size is approved for your vehicle or suitable for your driving needs. A tyre that can be described using a particular size code is not automatically suitable for every vehicle with a similar wheel diameter.

Before fitting a different tyre size, check the vehicle manufacturer’s specifications and get advice from a qualified tyre professional. Changes to tyre size can affect vehicle operation and may have safety, insurance or regulatory implications. Requirements can vary by state or territory, so check the rules that apply where the vehicle is registered.

Why should I check the tyre placard?

The tyre placard is a useful starting point because it lists information selected for the vehicle by its manufacturer. It can identify the recommended tyre size and the correct cold inflation pressures for different loading conditions. The pressure printed on a tyre sidewall is not a substitute for the vehicle manufacturer’s recommended pressure.

If you are unsure how to read the placard, or the information appears to differ from the tyres on the vehicle, ask a qualified tyre professional to check it. Do not rely on a size calculator alone to make a fitting or inflation decision.

Are tyre-size calculator results exact?

Calculator results are estimates derived from the size information entered. Tyre size markings describe nominal dimensions, and actual measurements can vary between tyre models. The calculator also depends on the accuracy of the information supplied. Check that you have entered the complete size correctly and use the result as a guide to understanding the specification.

For a vehicle-specific fitting decision, consult the manufacturer’s information and a qualified tyre professional. The calculator is a convenient way to explore tyre dimensions, but it cannot replace a physical inspection or professional advice.