Tire Size Conversion Calculator

Convert a tire size from metric to inches by entering the size below. Or, select the option to convert a tire size from inches to metric.

By

Joe Sexton

Joe is the creator of Inch Calculator and has over 20 years of experience in engineering and construction. He holds several degrees and certifications.

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How to Convert a Tire Size From Metric to Inches

To convert a tire size measured using metric notation to the size in inches, or SAE notation, can be done using a few formulas.

A metric tire size is composed of the section width in millimeters, the aspect ratio, and the rim diameter in inches. An inch tire size is the tire diameter in inches, the section diameter in inches, and the rim diameter in inches.

To convert from metric to inch sizes then, we need to do a few things.

First, we need to convert the section width from millimeters to inches. This can be done by dividing the number by 25.4. You can also use our millimeters to inches conversion calculator.

Next, we need to find the tire diameter in inches. You can use our tire size calculator to find the tire size, or you can use a few formulas.

sidewall = section width × (60 ÷ 100)
diameter = (sidewall × 2) + rim diameter

Thus, the wheel diameter is equal to the section width in inches times the aspect ratio divided by one hundred, times two, plus the rim diameter.

To finish the conversion let’s put it all together by composing the new size as the diameter “X” section width “R” rim diameter.

How to Convert a Tire Size From Inches to Metric

To convert a tire size from inches to metric the steps above need to be reversed.

Start by converting the section width from inches to millimeters by multiplying by 25.4. You can also use an inches to millimeters conversion calculator.

Then, solve the aspect ratio using the following formulas.

sidewall = (tire diameter – rim diameter) ÷ 2
aspect ratio = sidewall ÷ section width × 100

So, the aspect ratio is equal to the tire diameter minus the rim diameter, divided by two, divided by the section width, times one hundred.

Then, compose the metric tire size as the section width “/” aspect ratio “R” rim diameter.