Standard to Metric Tire Converter

What Is a Standard to Metric Tire Converter?

A standard to metric tire converter translates inch-format tire sizes into the equivalent metric designation used on most modern vehicles. Standard (inch) sizing, commonly written as 31x10.5R15, expresses overall tire diameter, section width, and rim diameter all in inches. Metric sizing, such as 265/75R15, expresses section width in millimeters and aspect ratio as a percentage, with rim diameter in inches. The two formats describe the same physical dimensions using different conventions.

This converter is used by truck and SUV owners shopping for replacements across different markets, off-road enthusiasts comparing tire catalogs from US and international brands, mechanics cross-referencing fitment charts, and anyone who needs to match an inch-format tire to a metric equivalent for ordering, mounting, or warranty documentation. The converter calculates both the exact metric value and the nearest standard metric size available from major tire manufacturers. To go the other direction and convert a metric size to its inch equivalent, use our metric to standard tire converter.

How the Standard to Metric Tire Converter Works

Entering Standard Tire Dimensions

Use the Standard Format tab to enter the tire code exactly as it appears on the tire sidewall, such as 33x12.5R15 or 35x12.5R17. The converter parses the overall diameter, section width, and rim diameter from the code automatically. Alternatively, switch to Manual Entry to type each dimension separately. The converter accepts any combination of inch measurements and handles decimal values such as 10.5 or 12.5 for section width.

Understanding the Metric Equivalent

The result shows two metric values. The exact metric equivalent converts your precise inch dimensions into millimeters and percentage without rounding. The nearest standard metric size rounds the section width to the nearest 5 mm and the aspect ratio to the nearest 5%, which matches the increment steps used in commercial tire production. Most tire searches and catalog lookups use the rounded standard size, so the converter shows both to cover exact technical work and practical shopping use cases.

How to Read Standard (Inch) Tire Sizes

Overall Diameter

The first number in a standard tire code is the overall diameter of the inflated, mounted tire measured from the ground to the top of the tread in inches. A 33x12.5R15 tire has an overall diameter of 33 inches. This number directly tells you the ground clearance benefit and vehicle height change when fitting that tire. Larger diameter tires raise the vehicle and increase ground clearance, which is why off-road builds typically focus on this number first. In the metric format, overall diameter is not stated directly but must be calculated from the width, aspect ratio, and rim diameter.

Section Width

The second number is the section width of the tire in inches, measured at the widest point of the inflated tire from sidewall to sidewall. In 33x12.5R15, the section width is 12.5 inches. Wider tires provide a larger contact patch for traction in mud and sand but require wider fender clearance and may need wheel spacers or a body lift. In the metric format, section width is the first number and is expressed in millimeters, so a 12.5-inch section width converts to approximately 317.5 mm, rounded to the standard 315 mm in commercial production.

Rim Diameter

The number after the R indicates the rim diameter in inches. Both standard and metric tire formats use the same inch value for rim diameter, so a 33x12.5R15 and its metric equivalent 315/70R15 both mount on a 15-inch wheel. The R indicates radial construction in both formats. Larger rim diameters are common in modern truck builds and performance off-road setups because they allow larger brake rotors and lower-profile tires with stiffer sidewalls for better handling on pavement.

How Standard Maps to Metric

The conversion from standard (inch) to metric uses three straightforward formulas. Using 33x12.5R15 as an example:

The rounding to the nearest 5 mm for section width and nearest 5% for aspect ratio reflects how tire manufacturers produce and catalog tires. Exact conversion values like 317.5/72R15 do not exist as commercial products, but 315/70R15 is a widely available size with nearly identical dimensions.

Common Standard to Metric Tire Conversions

Popular standard inch tire sizes with their metric equivalents. Rounded column shows the nearest available commercial metric size.

When Standard Tire Sizes Are Used

Off-Road and Truck Tires

Standard inch sizing dominates the off-road and light truck tire market in North America. Brands including BF Goodrich, Nitto, Toyo, and Mickey Thompson list most of their off-road tire lines in inch format because truck and Jeep owners think in terms of overall tire height rather than section width and aspect ratio. A Jeep Wrangler owner planning a build talks about fitting 35s or 37s, referring to overall diameter. The standard format communicates this directly. Most off-road tire fitment guides, forum discussions, and aftermarket parts catalogs use inch sizing for 4x4 and off-road applications, making it the practical standard in that segment even decades after the metric format became dominant for passenger cars. According to Tire and Rim Association sizing standards, both formats are recognized but serve different market applications.

Classic and Vintage Vehicles

Many classic and vintage vehicles were originally fitted with bias-ply tires measured in inch format. Restorers and enthusiasts maintaining these vehicles often need to convert older inch-format tire specifications into modern metric equivalents to find radial replacements with correct fitment. Classic trucks from the 1960s through 1980s, pre-metric era passenger cars, and specialty vehicles from before metric tire standardization all use inch-format tire specifications. Converting these to metric enables sourcing from the broader modern tire market while maintaining the correct overall diameter and section width for wheel well clearance and handling character. NHTSA tire safety and sizing information provides guidance on ensuring any tire replacement meets safety requirements.

Standard to Metric Examples

Example 1 - Convert 31x10.5R15

31x10.5R15 is one of the most common off-road tire sizes for stock or mildly lifted Jeeps and light trucks. To convert:

• Section Width: 10.5 in × 25.4 = 266.7 mm → rounded to 265 mm
• Sidewall: (31 − 15) ÷ 2 = 8 in = 203.2 mm
• Aspect Ratio: (8 ÷ 10.5) × 100 = 76.2% → rounded to 75%
• Metric equivalent: 265/75R15

265/75R15 is a widely available tire size sold by most major manufacturers in all-terrain, mud-terrain, and highway configurations. This makes the conversion useful for shopping across brands that catalog by metric size. The SEMA aftermarket tire and wheel standards document covers fitment guidance for metric and inch tire sizing interoperability.

Example 2 - Convert 33x12.5R17

33x12.5R17 is popular on lifted half-ton trucks and Jeep Wranglers with 17-inch aftermarket wheels. The conversion:

• Section Width: 12.5 in × 25.4 = 317.5 mm → rounded to 315 mm
• Sidewall: (33 − 17) ÷ 2 = 8 in = 203.2 mm
• Aspect Ratio: (8 ÷ 12.5) × 100 = 64% → rounded to 65%
• Metric equivalent: 315/65R17

315/65R17 is a standard size available from all major off-road tire manufacturers. When searching tire retailers that use metric sizing, 315/65R17 returns the same physical tire as 33x12.5R17 within the same product line. Always verify the overall diameter match when comparing sizes across formats to ensure correct fitment. You can also use our tire size converter to check metric tire dimensions and compare sizes side by side.

FAQ