Tire Size Calculator

Convert any tire size between metric and standard (flotation) sizing. Type a size such as 285/70R17 or 35x12.50R17, or pick one from the lists, and the calculator shows its height, width, sidewall, circumference and revolutions per mile in inches and millimeters, plus the closest size in the other system.

Convert a tire size

Metric to inches and inches to metric

Type any tire size

Convert

Try: 285/70R17 315/70R17 325/60R20 33x12.50R15 35x12.50R17 37x12.50R17

Metric size

Width (mm)155165175185195205215225235245255265275285295305315325335345355365375385395 Aspect ratio25%30%35%40%45%50%55%60%65%70%75%80%85% Wheel13"14"15"16"16.5"17"18"19"20"22"24"26"

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Standard (flotation) size

Height26"27"28"29"30"31"32"33"34"35"36"37"38"39"40"41"42"43"44" Width7.50"8.50"9.50"10.50"11.50"12.50"13.50"14.50"15.50"16.50"17.50" Wheel13"14"15"16"16.5"17"18"19"20"22"24"26"

How metric and standard tire sizes work

A metric size such as 285/70R17 gives the section width in millimeters (285), the sidewall height as a percentage of that width (70 percent) and the wheel diameter in inches (17). The height is not printed, so it has to be worked out: 285 ÷ 25.4 = 11.2 inches wide, 11.2 × 0.70 = 7.9 inches of sidewall, and two sidewalls plus the 17-inch wheel make a tire about 32.7 inches tall.

A standard or flotation size such as 35x12.50R17 states the dimensions directly: 35 inches tall, 12.5 inches wide, on a 17-inch wheel. Converting it to metric means finding the metric width and aspect ratio that land closest to that height and width, which is why the calculator lists the three closest metric sizes rather than one exact answer.

The formula behind every figure is height = 2 × width × aspect ÷ 100 ÷ 25.4 + wheel diameter. Circumference is the height × 3.1416, and revolutions per mile are 63,360 inches ÷ the circumference. These are calculated, unloaded figures: a tire’s published height can sit a few tenths of an inch above or below them, and a tire under load rolls a little shorter, so use the maker’s size chart for final clearance checks.

Tire size conversion chart

Common off-road and light truck sizes, calculated with the same formula and rounded to one decimal place. The closest flotation size is the nearest whole-inch height with the nearest standard width; the closest metric size is the width and aspect ratio that best match the flotation height and width.

Planning a bigger tire

Going up a size changes more than the look of the truck. A taller tire reads slower on the speedometer, lowers the effective gear ratio and needs room to turn and flex. For the full math with worked examples, speedometer error and regearing rules of thumb, read 325/60R20 in Inches: Tire Size Math Made Simple. When you know the size you are after, browse off-road tires, wheels and tires and lift kits and suspension to make it fit.