Metric Tire Calculator: Convert Tire Size to Inches

A metric tire calculator converts a tire’s metric size into its approximate overall diameter and width in inches. Enter the tire’s section width, aspect ratio, and wheel diameter; the calculator returns the tire’s calculated width and diameter, plus its sidewall height.

Formula

A metric tire size such as 225/65R17 describes a nominal section width of 225 millimeters, an aspect ratio of 65%, and a tire designed for a 17-inch-diameter rim. The calculator applies these relationships:

• Sidewall height (in) = width (mm) × aspect ratio (%) ÷ 2,540
• Overall diameter (in) = wheel diameter (in) + 2 × sidewall height (in)
• Section width (in) = width (mm) ÷ 25.4

In the calculator, width is the tire’s nominal section width in millimeters, ratio is the aspect-ratio number (for example, enter 65 for 65%), and rim is the wheel diameter in inches. The number 2,540 converts the width-times-percentage product into inches: 25.4 millimeters equal one inch, and the ratio is entered as a percent rather than a decimal.

Worked Example

Using the defaults, the tire is 225/65R17. Its nominal sidewall height is 225 × 65 ÷ 2,540 = 5.758 inches, which rounds to 5.76 inches. The overall diameter is 17 + 2 × 5.758 = 28.516 inches, or 28.52 inches to two decimal places. Its nominal section width is 225 ÷ 25.4 = 8.86 inches.

How to Read a Metric Tire Size

In a marking such as 225/65R17, the first number is the nominal width from sidewall to sidewall, in millimeters. The second number is the sidewall height as a percentage of that width, not a measurement in millimeters. The letter R means radial construction; it does not mean “rim.” The final number gives the wheel diameter in inches.

For the example, 65% of 225 mm gives a sidewall height of 146.25 mm, or about 5.76 inches. There are two sidewalls between the wheel’s bead-seat diameter and the tire’s outer tread, so the estimate adds twice that height to the 17-inch wheel diameter.

When to Use the Conversion

Metric-to-inch conversion helps when comparing tire sizes, estimating a replacement tire’s diameter, or checking whether a proposed size is near the original. Diameter can also help estimate how far a tire travels per revolution. It is a geometric comparison, not proof that a tire will fit a particular vehicle or wheel.

Compare the calculated diameter with the tire currently fitted or with the vehicle maker’s specified size. A size with a similar diameter may still differ in width, load capacity, speed rating, wheel compatibility, or clearance. Check the vehicle placard and owner’s manual, and confirm that the tire’s service description and wheel requirements are suitable.

Why the Calculator Is an Estimate

The calculation uses the dimensions encoded in the size designation. Actual mounted dimensions can vary with tire design, tread depth, measuring rim width, inflation pressure, load, and manufacturing tolerances. A tire’s published specifications, when available, are more useful for precise clearance comparisons than a size-code calculation alone.

The computed width is the nominal section width, not necessarily the width of the tread. It is also not a guarantee of the installed tire’s widest point: sidewall shape and the wheel’s width affect the mounted profile. For close-fitting applications, verify measurements and clearances rather than relying only on the nominal conversion.

Common Mistakes and Limits

• Treating the aspect ratio as millimeters: 65 means 65% of the section width. It is not a 65 mm sidewall.
• Using the wrong wheel units: the last number in a metric tire size is already in inches. Do not convert it from millimeters.
• Forgetting the second sidewall: overall diameter adds twice the sidewall height, not once.
• Confusing width and diameter: the first size number estimates section width; it does not give the tire’s outer diameter.
• Rounding too soon: keep the full sidewall result through the diameter calculation, then round the final display.

This formula does not calculate revolutions per mile, actual loaded radius, vehicle speedometer error, or fitment clearance. Those require additional measurements or specifications. A tire’s loaded radius is smaller than its unloaded radius because the tire flexes under the vehicle’s weight.

Frequently Asked Questions

How do I convert 225/65R17 to inches?
Divide 225 mm by 25.4 for a nominal width of 8.86 inches. Calculate the sidewall as 225 × 0.65 ÷ 25.4, or about 5.76 inches, then add two sidewalls to the 17-inch wheel. The estimated overall diameter is 28.52 inches.

What does the 65 mean in a tire size?
It is the aspect ratio: the sidewall height is 65% of the nominal section width. For a 225 mm tire, that is 146.25 mm, or approximately 5.76 inches.

Is tire width the same as tread width?
No. The size-code width is nominal section width, measured across the tire’s sidewalls under defined conditions. Tread width is a separate dimension and may be narrower.

Will a tire with the same diameter fit my vehicle?
Not necessarily. Width, wheel diameter and width, load and speed ratings, clearance, and vehicle specifications all matter. Use the vehicle maker’s approved sizes and check fitment details before changing tire size.

Why is my tire’s measured diameter different from the calculator?
The formula estimates dimensions from the size code. Tire construction, wear, pressure, load, measuring rim, and manufacturing variation can change the actual measurement.

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in width | Tire width | mm | 225 | 0
in ratio | Aspect ratio | % | 65 | 0
in rim | Wheel diameter | in | 17 | 0
out diameter | Overall tire diameter | in | sqrt((rim + 2 * width * ratio / 2540)^2) | 2
out section | Section width | in | width / 25.4 | 2
out sidewall | Sidewall height | in | width * ratio / 2540 | 2