Tire Size Conversion: Metric Sizes, Inches and Flotation Markings

Metric and flotation sizes answer different questions

A metric tire marking such as 285/70R17 gives nominal width in millimeters and a sidewall-to-width percentage. Its outside diameter must be calculated. A flotation marking such as 33x12.50R15 gives nominal outside diameter and section width in inches directly, followed by the matching rim diameter. Reading 33 as an aspect ratio would produce a completely different and incorrect result.

Converting a size helps you compare its proportions with another system. It does not rename the tire or establish that another marked size is an interchangeable replacement. Keep the original size string in your notes so that a nominal calculation cannot be mistaken for a manufacturer specification.

Convert a metric size with one exact unit relationship

One inch equals exactly 25.4 mm. For 285/70R17, divide 285 by 25.4 to obtain a nominal width of approximately 11.22 inches. The nominal sidewall is 285 × 0.70 = 199.5 mm, or about 7.85 inches. Add two sidewalls to the 431.8 mm rim diameter to obtain 830.8 mm, or approximately 32.71 inches overall.

Retain full precision during the calculation and round only the displayed result. Rounding the width to 11 inches before calculating sidewall height changes the answer unnecessarily. The tire size calculator uses the full input values, so a comparison can be reproduced without repeating several manual conversions.

A second example, 245/75R16, gives 183.75 mm sidewalls and 773.9 mm overall diameter. That is approximately 30.47 inches tall and 9.65 inches wide in the nominal model. It should not be described as an exact 31-inch tire simply because a store groups it with nearby outside diameters.

Decode flotation without inventing an exact metric equivalent

For 33x12.50R15, multiply the first two dimensions by 25.4: nominal diameter is 838.2 mm and nominal section width is 317.5 mm. The rim is 381 mm across its nominal diameter. The remaining diameter is 457.2 mm, which gives a nominal sidewall height of 228.6 mm after dividing by two.

You can express that geometry as a width and a calculated ratio, but the ratio may not match an available standardized tire marking. Even when a nearby metric size exists, its diameter, width, approved rim range and service specification must be checked independently. “Closest in inches” is a numerical comparison, not a product substitution rule.

The table above also shows why a size described casually as a 33-inch tire is not necessarily the same as 33x12.50R15. The 285/70R17 example is nominally about 0.29 inches shorter and 1.28 inches narrower, and it requires a different rim diameter.

Use the conversion to build a useful comparison

First decode the current and proposed markings separately. Then compare diameter, width and sidewall height using the same units. After that, obtain product data for the exact tire models and write down the measuring rim, actual published diameter and approved rim widths. A nominal conversion cannot reveal an unusually wide shoulder or a protective sidewall rib.

Finally, evaluate vehicle requirements and clearances with the wheels included. A narrower nominal tire can still be mounted farther outward if wheel offset changes. A matching nominal diameter also says nothing about load capacity or suitability for the vehicle. This sequence keeps the conversion useful without asking it to answer questions that depend on the actual assembly.

Common questions