Tire Size Diameter Calculator

Tire size diameter calculator results compare the nominal dimensions of two metric radial sizes. Use the result to understand the geometric change, then confirm the replacement size, wheel fit, load capacity, and clearance for the exact vehicle.

What the Comparison Shows

• Nominal width, sidewall height, diameter, and circumference.
• Theoretical revolutions per mile and kilometer.
• Ride-height change based on half the diameter difference.
• A geometric speedometer estimate at the indicated speed you choose.

The calculator supports common metric radial markings, including P and LT prefixes, ZR sizes, and many service-description suffixes. It does not support flotation sizes such as 31x10.50R15.

How Metric Tire Size Becomes Diameter

In 225/45R17, 225 is nominal section width in millimeters. The sidewall height is 45 percent of that width. R means radial construction, and 17 is the wheel's bead-seat diameter in inches. Overall diameter is two sidewalls plus the wheel diameter after converting inches to millimeters. NHTSA's tire sidewall guide explains these markings.

Why Nominal and Rolling Size Differ

The sidewall formula gives a nominal unloaded diameter. Actual dimensions and distance per revolution change with tire model, approved rim width, inflation, load, tread depth, and manufacturing tolerance. Use the tire maker's measured diameter or revolutions-per-mile specification when calibration accuracy matters.

Size Math Does Not Confirm Fit

A close diameter does not prove that the tire will clear the body and suspension or fit the wheel. Width, wheel offset, rim-width range, steering angle, suspension travel, chains, and brake clearance also matter. Changing overall diameter can affect gearing, speed display, traction control, stability control, and braking. Michelin recommends checking vehicle-maker restrictions before using an alternate size in its plus-sizing guidance.

Keep Load, Speed, and Pressure Requirements

Check the driver-door tire placard and owner's manual, not just the old tire. A replacement must meet the vehicle's load needs and use a suitable speed rating, construction, and service type. Do not use the maximum pressure molded on the tire as the normal vehicle pressure. NHTSA provides tire selection and maintenance guidance on its TireWise safety page.