How to Read Tire Size Numbers (Complete Guide)

Understanding Metric Tire Sizes

The most common tire size format is metric, like P265/70R17. Here's what each part means:

  • P — Tire type. P = Passenger, LT = Light Truck, ST = Special Trailer, C = Commercial
  • 265 — Section width in millimeters (the widest point of the tire when mounted)
  • 70 — Aspect ratio (sidewall height = 70% of the width, so 265 × 0.70 = 185.5mm)
  • R — Construction type. R = Radial (nearly all modern tires). B = Bias-ply, D = Diagonal
  • 17 — Rim diameter in inches

Flotation Tire Sizes

Common on trucks and off-road vehicles, flotation sizes use inches: 33x12.50R15

  • 33 — Overall tire diameter in inches
  • 12.50 — Section width in inches
  • R — Radial construction
  • 15 — Rim diameter in inches

Motorcycle Tire Sizes

Motorcycle tires use a similar metric format: 120/70ZR17

  • 120 — Width in millimeters
  • 70 — Aspect ratio
  • Z — Speed rating (Z = 149+ mph)
  • R — Radial construction
  • 17 — Rim diameter

Calculating Overall Diameter

For a 265/70R17 tire:

  1. Width: 265mm ÷ 25.4 = 10.43 inches
  2. Sidewall height: 10.43 × 0.70 = 7.30 inches
  3. Overall diameter: 17 + (7.30 × 2) = 31.61 inches

Use our tire size calculator to do this instantly.