Tire Size Calculator and Conversion Chart: Metric to Inches
Tire size codes pack several measurements into a short string, but they rarely tell you directly how tall or wide a tire is in inches. Learning to calculate tire size yourself makes calculators and conversion charts easier to trust and easier to question. The math is straightforward, yet the results are nominal estimates, and choosing a different size still depends on vehicle-specific guidance.
Read a Metric Tire Size and Calculate Its Dimensions
A size such as 225/60R17 contains four nominal pieces of information. The 225 is section width in millimeters, the 60 is aspect ratio, meaning sidewall height equals 60 percent of that width, the R denotes radial construction, and 17 is rim diameter in inches. Because the code mixes units, convert before combining anything. Sidewall height in inches equals width times aspect ratio divided by 100, then divided by 25.4. Overall diameter equals rim diameter plus twice the sidewall height, since sidewall sits above and below the wheel.
Applied to 225/60R17, the width converts to about 8.86 inches, the sidewall works out to 135 millimeters or roughly 5.31 inches, and the overall diameter lands near 27.63 inches. Carry full precision through each step and round only the final figures, because rounding early lets small errors stack. Treat those results as nominal estimates rather than measurements. Tread design, the rim width used for measuring, inflation pressure, vehicle load, and tread wear all shift real dimensions, so two tires with identical codes can differ slightly.
Convert Metric Dimensions to Inch-Flotation Notation
Converting a metric size to inches mostly means dividing millimeter values by 25.4; the rim diameter needs no conversion because metric codes already state it in inches. Inch-flotation sizes, common on light trucks and off-road tires, present the result differently. In 31x10.50R15, 31 is nominal overall diameter in inches, 10.50 is nominal section width in inches, R indicates radial construction, and 15 is rim diameter. The format leads with overall height, which is often the number buyers care about most when comparing larger tires.
Once a metric size has been converted, compare its diameter and width with flotation sizes to find an approximate counterpart. The math also runs backward: 31x10.50R15 implies a sidewall of about 8 inches, half of 31 minus 15, and a width of roughly 267 millimeters, giving an aspect ratio near 76 percent. A close numerical match, however, is only arithmetic. It says nothing about whether a real tire exists in that size, what load it can carry, or whether it suits a particular vehicle.
Read a Tire Size Conversion Chart
A useful conversion chart lists each tire size alongside section width, sidewall height, overall diameter, and circumference, with a unit label on every column so inches and millimeters are never confused. Circumference equals pi times diameter, and estimated revolutions per mile equal 63,360, the number of inches in a mile, divided by circumference in inches. For a 27.63-inch tire, that gives about 86.8 inches and roughly 730 revolutions. Published rolling figures usually differ, because a loaded tire flattens at the contact patch and rolls on a smaller effective radius.
To use a chart, find the size row and read across for its converted dimensions. Neighboring rows can mislead, because aspect ratio is a percentage of width rather than a fixed length. A five-point step at 225 millimeters changes sidewall height by about 0.44 inch, while the same step on a wider tire changes it more. Good charts apply one rounding rule throughout and label entries as calculated nominal dimensions. When exact height, width, or revolutions per mile matter, the manufacturer's published specifications for the specific tire are the better reference.
Calculate the Dimensional Change From the Original Size
The right starting point is the size on the vehicle's tire placard, commonly found on the driver's door jamb, along with any alternative sizes the vehicle manufacturer lists. The tires currently mounted may not reflect that baseline; a previous owner or shop could have installed a different size, and calculating from the wrong reference hides the true change. With the original and proposed sizes entered into a calculator, look for signed differences in diameter, width, sidewall height, and circumference, where a plus or minus sign shows direction as well as magnitude.
Percentage change in diameter equals the proposed diameter minus the original diameter, divided by the original diameter, multiplied by 100. A positive result means the new tire is taller and covers more ground per revolution; a negative result means it is shorter and covers less. Going from 27.63 to 28.18 inches, for example, is about a 2 percent increase. Matching diameter closely does not make a size an approved replacement, though. Load index, speed rating, and wheel fitment are separate questions that the diameter figure cannot answer.
Determine Whether a Diameter Change Is Acceptable
No universal percentage defines an acceptable diameter change. A figure of roughly 3 percent circulates widely as a screening convention, useful for ruling out obviously large departures, but it is not approval from the vehicle manufacturer. Before settling on an alternative size, check the manufacturer's tire guidance, the load capacity the vehicle requires, the necessary speed rating, and any stated restrictions, such as sizes excluded for use with traction devices. A tire that passes the percentage test yet carries less load than the original specification is not a suitable substitute.
Two different changes are easy to confuse. Changing all four tires to a new size shifts the vehicle's baseline rolling circumference, while mixing sizes or wear levels between axles creates a mismatch the drivetrain must absorb. All-wheel-drive and four-wheel-drive systems can have separate, tighter limits on circumference differences. Antilock braking, stability control, and other systems that rely on wheel-speed signals may also respond to changed rotation rates. When suitability remains uncertain, a qualified tire professional with vehicle-specific information should make the call rather than a calculator alone.
Estimate Speedometer, Ride Height, and Clearance Effects
If the speedometer was calibrated for the original size, actual speed is approximately indicated speed multiplied by the proposed diameter and divided by the original diameter. With a hypothetical 2 percent diameter increase, an indicated 60 mph corresponds to about 61.2 mph. The odometer follows the same pattern, tending to underrecord distance because each revolution covers more ground than the system assumes. These remain nominal figures. Speedometers already carry some inherent error, and real rolling behavior differs from calculated diameter, so treat the result as an estimate rather than a correction factor.
Static ride height changes by roughly half the diameter change, since only the radius below the axle centerline lifts the vehicle. A 2 percent increase on a 27.63-inch tire adds about 0.55 inch of diameter and therefore roughly 0.28 inch of height, though loaded radius and sidewall deflection alter the actual result. A larger radius and wider section also reduce space to fenders, liners, and suspension parts. Static numbers cannot confirm clearance through full steering lock and suspension travel, so have a tire professional assess the fit.