How to Convert Metric Tire Size to Inches

A metric tire size like 265/70R17 converts to inches through three numbers: section width, sidewall height, and overall diameter. Width in inches = width in millimeters / 25.4. Sidewall height in inches = (width in mm x aspect ratio / 100) / 25.4. Overall diameter in inches = (rim diameter in inches x 25.4 + 2 x width in mm x aspect ratio / 100) / 25.4. For 265/70R17 that works out to a 10.43 in width, a 7.30 in sidewall, and a 31.61 in overall diameter.

The size code 265/70R17 packs three numbers into one string. 265 is the section width in millimeters, measured sidewall to sidewall across the widest part of an inflated, unloaded tire. 70 is the aspect ratio: the sidewall height as a percentage of that width, so a lower number means a shorter sidewall relative to the tire's width. 17 is the wheel rim diameter in inches, not the tire's own overall diameter -- rim diameter is already in inches, which is why that part of the code needs no conversion. Width and sidewall height start out in millimeters because that is how the size is defined and printed on the tire; converting them to inches is a unit change, not a different measurement.

Three conversions cover the whole size code:

• Width in inches = width in mm / 25.4
• Sidewall height in inches = (width in mm x aspect ratio / 100) / 25.4
• Overall diameter in inches = (rim diameter in inches x 25.4 + 2 x width in mm x aspect ratio / 100) / 25.4

The rim term (rim diameter x 25.4) puts the rim into millimeters first, then the whole diameter converts back to inches by dividing by 25.4 once at the end. The 2x in front of the sidewall term is there because the tire has a sidewall above the rim and another below it -- the rim sits in the middle of the overall diameter, not at one edge of it.

Running seven common sizes through the same three conversions gives the numbers below, calculated directly from width, aspect ratio and rim diameter -- or, for the flotation size in the last row, from the overall diameter and width already printed on the tire in inches (see the note after the table).

Run any other metric size through the tire size calculator for the same three numbers, plus a side-by-side comparison against a second size.

The last row above is a flotation size, written as width x diameter, rim rather than width/aspect, rim -- 33X12.50R17 means a nominal 33 in overall diameter and a 12.50 in section width, with both numbers already in inches. Run through the same geometry, that label converts to exactly 33.00 in (838.2 mm), because the label is the input here, not a value derived from a metric width and aspect ratio.

What the label does not capture is manufacturing variance: two tires stamped 33X12.50R17 from different manufacturers, or from different molds at the same manufacturer, commonly measure a few tenths of an inch apart once built, and inflation pressure and tread wear shift the measured diameter further from the nominal number over time. The geometry above gives the label's own arithmetic, not a manufacturer-measured value for one specific tire.

Two mix-ups come up often when reading a metric size in inches:

• Width mistaken for overall diameter. The first number (265, 285, 225...) is the section width in millimeters, not the tire's overall size -- 265/70R17 and 225/45R17 both start with a three-digit width, but their overall diameters (31.61 in and 24.97 in above) are more than 6 in apart, because aspect ratio and rim diameter both change the total diameter far more than width alone does.
• Rounding a calculated diameter to a whole inch. 31.61 in rounds to 32 in in casual conversation, and 34.36 in rounds to 34 in, but the calculated value is the one the formula above actually produces -- rounding to a whole number is a convenience for talking about a tire, not a more accurate figure.

Every number on this page comes from the same three conversions applied to the width, aspect ratio and rim diameter printed on a tire's sidewall (or, for a flotation size, its printed overall diameter and width). This is nominal geometry, not a manufacturer-measured value -- the actual diameter of a built tire varies by manufacturer, mold, inflation pressure and tread wear, usually by a small margin rather than a large one, but enough that a spec sheet from the tire's manufacturer is the more precise source for one specific tire. See the site disclaimer for the full limits, and how to read a tire sidewall for what each part of the size code refers to before converting it.