Tire sizes

Tire Size Converter [Metric to Inches]

Tire sizes, often confusing due to their variety, follow a standard convention revealing important dimensions. Knowing how to read and convert these sizes is crucial for selecting appropriate tires.

To simplify this process, I’ve created a Tire Size Converter Calculator, enabling effortless conversions between metric and inch-based sizes.

Tire Size Converter Calculator

Tire Size Converter

Tire Size Converter

Convert Metric Tire Size to Inches

Tire Sizing Convention

Tire sizes are made up of a combination of letters and numbers that indicate a tire’s key dimensions. Here is an overview of what the different components of a tire size mean:

  • Section Width: The first three-digit number indicates the tire’s section width in millimeters. This measures the width of the tire from sidewall to sidewall.
  • Aspect Ratio: This two-digit number shows the aspect ratio, which is the tire’s section height as a percentage of its width. So a 50 aspect ratio means the tire’s section height is 50% of its section width.
  • Construction Code: This letter indicates the tire’s construction method. R stands for radial, the most common type of passenger tire.
  • Wheel Diameter: This two-digit number is the diameter of the wheel that the tire is designed to fit, in inches.
  • Load Index: A two or three-digit code that specifies how much weight a tire can support (its load carrying capacity).
  • Speed Rating: A letter code that indicates the maximum speed a tire is rated for. Common speed ratings are S, T, H, V, W, Y, Z.

For example, a tire marked P215/65R15 95H would have a section width of 215mm, an aspect ratio of 65%, radial construction, is designed to fit a 15-inch wheel, has a load index of 95 (equivalent to 1,521 pounds), and speed rating of H (130 mph).

Tire Size Conversion Chart: Metric to Imperial

Common Tire Size (P-Metric) Width (inches) Aspect Ratio Wheel Diameter (inches)
P215/65R15 8.46 65 15
P225/70R16 8.85 70 16
P235/75R15 9.25 75 15
P245/45R20 9.64 45 20

Why Tire Sizing Matters

Understanding manufacturer tire sizing is important because the tire dimensions must match the requirements of your vehicle.

Installing a tire that is too small or too large can impact the vehicle’s handling, speedometer accuracy, and wheel wells.

The tire size information is located on the sidewall of the tire. Vehicle owners should consult their owner’s manual or vehicle door jamb for the OE (original equipment) tire size recommended by the manufacturer.

Considerations When Converting Sizes

When converting tire sizes, keep the following considerations in mind:

  1. Small variations in section widths and diameters are usually fine but avoid large differences that could impact wheel wells.
  2. Don’t exceed the recommended load and speed ratings from the vehicle manufacturer.
  3. Measure the actual diameter of the current tires and compare to new size options.
  4. Input the current tire size and the new size to see how the rolling diameter differs (affects the speedometer). Consult a tire retailer or auto technician if you have questions about compatibility.

Following the tire size recommendations from the vehicle maker is always the best approach. But tire size converters make it straightforward to explore alternate tire sizes and make an informed selection.


Tire sizing can be confusing, given the different conventions and letter/number codes used.

Online tire size conversion tools (Like the I created above) simplify the process of switching between metric and inch-based sizes.

When converting sizes, take care to match key dimensions, load capacity, and speed rating

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