Tire Size Calculator

Width (mm)

Aspect Ratio (%)

Rim Diameter (in)

How to Read and Calculate Tire Sizes

Tire sizes look like "225/45R17". Each number tells you a specific dimension of the tire.

Breaking down the size code

225 — Tread width in millimeters
45 — Aspect ratio (sidewall height as a percentage of width). 45 means the sidewall is 45% of 225 mm.
R — Radial construction (nearly all modern tires)
17 — Rim diameter in inches

The calculations

Sidewall Height = Width × Aspect Ratio ÷ 100

Overall Diameter = (Sidewall × 2) + (Rim × 25.4) (25.4 converts inches to mm)

Circumference = Diameter × π

Worked example

225/45R17: Sidewall = 225 × 0.45 = 101.25 mm. Diameter = (101.25 × 2) + (17 × 25.4) = 202.5 + 431.8 = 634.3 mm (24.97 inches). Circumference ≈ 1,993 mm.

Why this matters

Changing tire size affects speedometer accuracy, ride height, handling, and gear ratios. A larger diameter tire rolls farther per revolution, making your speedometer read low. Always consult a tire professional before changing sizes.