Tyre Size Converter
Tyre markings follow standardized notation systems that encode width, aspect ratio, construction type, and rim diameter. Most modern cars use the metric system (e.g. 225/45R17), but understanding the notation and converting between formats is essential when selecting replacement tyres or comparing different sizes. The section width (the first number) is measured in millimetres from sidewall to sidewall. The aspect ratio (the number after the slash) expresses the sidewall height as a percentage of the width. The letter R indicates radial construction, the standard for car tyres. The final number is the rim diameter in inches. This converter translates metric notation into imperial dimensions (section width in inches, overall diameter in inches) and calculates the tyre's circumference, which determines how many rotations the tyre makes per mile and affects your speedometer calibration. All calculations follow the US Tire Manufacturers Association (USTMA) standards for tyre marking. Enter a metric tyre size in the format "225/45R17" and the calculator displays the section width, sidewall height, overall diameter, and circumference in both metric and imperial units. Use this converter when shopping for replacement tyres, calculating speedometer effects of a size change, or comparing tyres across different markets.
How tyre sizes are calculated
Metric tyre notation encodes all the information needed to calculate the tyre's dimensions. Given a size like 225/45R17:
- Section width (225 mm): The width of the tyre from sidewall to sidewall, measured in millimetres.
- Aspect ratio (45%): The sidewall height as a percentage of the section width. A 45 aspect ratio means the sidewall is 45% of 225 mm, or 101.25 mm tall.
- Rim diameter (17 inches): The diameter of the wheel the tyre fits on.
- Overall diameter: The total outer diameter, calculated as the rim diameter plus twice the sidewall height: (17 in x 25.4 mm/in) + 2 x 101.25 mm = 431.8 mm + 202.5 mm = 634.3 mm, or 24.98 inches.
- Circumference: The distance around the tyre's outer edge, calculated as pi times the overall diameter: 3.14159 x 634.3 mm = 1,992.7 mm or 78.45 inches.
All measurements are derived from the three numbers in the metric notation. Changing any of these values produces a different size with different overall diameter and circumference, which affects speedometer readings and fuel economy.
Tyre size converter: frequently asked questions
What does 225/45R17 mean?
In metric tyre notation: 225 is the section width in millimetres (sidewall-to-sidewall width), 45 is the aspect ratio (sidewall height as a percentage of section width), R indicates the radial construction (standard for nearly all car tyres), and 17 is the rim diameter in inches. So a 225/45R17 tyre has a 225 mm width, a sidewall height of 101.25 mm (225 x 0.45), and fits on a 17-inch diameter rim.
How do I convert between metric and imperial tyre sizes?
Metric sizes express width in millimetres; imperial sizes usually express width in inches. The converter multiplies section width in mm by 0.03937 to get inches. To find the overall diameter, add twice the sidewall height (in the same units) to the rim diameter. The calculator handles all conversions automatically.
What is the aspect ratio?
The aspect ratio is the ratio of the sidewall height to the section width, expressed as a percentage. A 45 aspect ratio means the sidewall is 45% as tall as the width is wide. Lower aspect ratios (like 30 or 35) produce a sportier, stiffer ride; higher aspect ratios (like 60 or 65) produce a more comfortable ride with thicker sidewalls.
What is tyre circumference used for?
Tyre circumference is the distance the tyre travels in one complete rotation. It is used to calculate speedometer accuracy, odometer calibration, and gear ratio effects. A larger-circumference tyre travels farther per rotation, which can affect speedometer readings if you change tyre sizes.
Can I use a different size tyre on my car?
Your car's owner manual specifies the approved tyre sizes and load ratings. Changing tyre size can affect speedometer accuracy, fuel economy, acceleration, and handling. Any size change must stay within the vehicle manufacturer's approved range and maintain the same load index and speed rating.
Official sources
- US Tire Manufacturers Association: USTMA tyre marking standards and technical specifications.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.