Tire Size Calculator Formula

Understand the math behind the tire size calculator. Each variable explained with a worked example.

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Formulas Used

Overall Diameter

overall_diameter = rim_diameter + 2 * sidewall_in

Circumference

circumference = (rim_diameter + 2 * sidewall_in) * pi

Sidewall Height

sidewall_height = sidewall_in

Revolutions Per Mile

revs_per_mile = 63360 / ((rim_diameter + 2 * sidewall_in) * pi)

Variables

VariableDescriptionDefault
width_mmTire Width(mm)225
aspect_ratioAspect Ratio(%)55
rim_diameterRim Diameter(in)17
sidewall_mmDerived value= width_mm * aspect_ratio / 100calculated
sidewall_inDerived value= sidewall_mm / 25.4calculated

How It Works

How to Read a Tire Size

A tire marked 225/55R17 means: 225 mm width, 55% aspect ratio (sidewall height as % of width), R = radial, 17-inch rim.

Formula

Sidewall = Width x Aspect Ratio / 100 (in mm)

Overall Diameter = Rim Diameter + 2 x Sidewall (converted to inches)

Circumference = Diameter x pi

Worked Example

Tire size 225/55R17.

width_mm = 225aspect_ratio = 55rim_diameter = 17
  1. 01Sidewall = 225 x 55/100 = 123.75 mm = 4.87 in
  2. 02Overall diameter = 17 + 2 x 4.87 = 26.74 in
  3. 03Circumference = 26.74 x 3.1416 = 84.0 in
  4. 04Revolutions per mile = 63,360 / 84.0 = 754

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