Sidewall height
Section width × aspect ratio ÷ 100
Example: 225 × 0.50 = 112.5 mm
Tire Size Decoder & Dimensions Calculator
Enter a tire size such as 225/50R17 98V XL to decode the sidewall marking and calculate theoretical unloaded tire dimensions.
Quick Summary
This tool decodes common tire size markings and calculates theoretical unloaded dimensions from the size designation. It can explain section width, aspect ratio, construction, rim diameter, load index, speed rating, and service descriptions such as XL or LT.
Calculated dimensions are theoretical. Actual measured tire dimensions vary by manufacturer, model, measuring rim width, inflation pressure, load, construction, tread depth, and measurement method.
Decode a Tire Size
How Tire Dimensions Are Calculated
For a metric tire such as 225/50R17, the section width is 225 mm. The aspect ratio is 50, meaning the sidewall height is 50% of the section width. Overall diameter is calculated by adding two sidewalls to the wheel diameter.
Overall diameter
Wheel diameter + 2 sidewalls
The wheel diameter is converted to the same unit before calculating the final diameter.
Why Actual Tires May Measure Differently
The output from this calculator is theoretical and unloaded. A real mounted tire may measure differently because of manufacturer design, approved measuring rim width, inflation pressure, load, tread design, tread depth, construction, and measurement method.
This tool does not approve fitment, load capacity, speed capability, roadworthiness, or compatibility with a specific vehicle. Always follow the tire manufacturer, vehicle manufacturer, and qualified service guidance.
Where to Find This Information
The tire size is molded into the tire sidewall. On many passenger tires it appears in a format like 225/50R17 98V XL. Light-truck tires may use markings such as LT265/70R17 121/118S, where the two load indexes identify single and dual tire load service.
Tire Size Decoder FAQ
Is this the same as a tire size comparison tool?
No. This tool decodes one tire size and calculates its theoretical unloaded dimensions. The Tire Size Comparison Tool compares two different sizes.
Does the calculator show actual measured tire dimensions?
No. It calculates theoretical dimensions from the size designation. Actual mounted and loaded dimensions vary by tire model, measuring rim width, inflation pressure, load, and construction.
What does XL mean on a tire size?
XL means Extra Load. It indicates the tire is designed to carry more load than a standard-load tire when inflated to the appropriate pressure.
What does 121/118S mean on a light-truck tire?
The first number is the single-tire load index and the second number is the dual-tire load index. The letter is the speed rating.