Formula for Calculating Tire Dimensions
Use this simple, repeatable formula to convert any modern P-metric tire size into section width and overall diameter. Below we show the math using 235/35R19 as the example, including rounding tips and links to our calculators.
Measurements
Key terms: the first number is nominal section width in millimeters (mm). The second number is the aspect ratio (sidewall height as a percent of section width). The last number is the wheel (rim) diameter in inches.
- Convert width to inches: section width (in) = width (mm) ÷ 25.4
- Sidewall height: sidewall (in) = section width (in) × aspect ratio
- Overall diameter (OD): OD (in) = 2 × sidewall (in) + rim diameter (in)
- Metric method: OD (mm) = 2 × width (mm) × aspect ratio + 25.4 × rim (in); then divide by 25.4 to get inches
Worked example (235/35R19):
- Section width (in) = 235 ÷ 25.4 = 9.25 inches (?9.3" rounded)
- Sidewall height = 9.25 × 0.35 = 3.24 inches
- Overall diameter = (3.24 × 2) + 19 = 25.48 inches ? round to 25.5"
Notes: Actual measured section width can vary slightly by tire brand/model and the rim width used. As a general guide, section width changes roughly ~0.2" for each 0.5" change in rim width. For quick comparisons and exact mm?inch conversions, use our Advanced Tire Size Calculator and Tire Size Comparison Calculator.
Clearance Checks
- Fender & suspension: Compare current OD to the new OD; larger diameters risk rubbing on fenders/liners, smaller diameters reduce ground clearance.
- Brake & strut clearance: Verify inner sidewall clearance on your current setup before upsizing width.
- Speedometer/ABS: Keep OD changes modest to avoid speedo error and drivetrain/ABS issues. Use the Plus/Minus Sizing Calculator to stay within common targets.
Safety & Load Ratings
- Load Index & Speed Rating: Choose tires that meet or exceed OEM specifications. Review our Tire Load & Speed Ratings.
- UTQG: Understand treadwear, traction, and temperature ratings with our UTQG guide.
- XL/LT: Match the correct construction (Standard, XL, or LT) for your vehicle and usage.
Professional Fitment Advice
Not sure which size is best? Start with our Wheel & Tire Size Calculator for dimensions and fitment planning. When you’re ready, shop Tires or build complete Wheel & Tire Packages. If you have a classic with non-standard markings, see Antique & Classic Tire Size Conversion or Contact Us for help.
Get Help (CTAs)
Reviewed by the Performance Plus Tire Fitment Team.
