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Tire Size Calculator

Calculate sidewall height, overall diameter, rolling circumference, and revolutions per km from a tire's width, aspect ratio, and rim size.

How it is calculated

A size code such as 205/55R16 gives the section width in millimetres, the aspect ratio in percent and the rim diameter in inches. The tool computes sidewall = width × aspect ÷ 100 in mm, then overall diameter = rim × 25.4 + 2 × sidewall, converting the rim from inches to mm and adding the sidewall twice (top and bottom). The rolling circumference is π × overall diameter, and revolutions per kilometre is 1,000,000 ÷ circumference (in mm).

All of these are geometric figures for an unloaded tire, shown to 0.1 mm, with revolutions rounded to a whole number. Real tires flatten slightly under load, wear down and grow with inflation, so the rolling circumference in practice is a little smaller. The tool does not check load index, speed rating or whether a size fits your wheel arch or rim width.

Worked example

For a 205/55R16 tire: width 205 mm, aspect 55%, rim 16 in. The sidewall is 205 × 55 ÷ 100 = 112.8 mm. The rim is 16 × 25.4 = 406.4 mm, so the overall diameter is 406.4 + 2 × 112.75 = 631.9 mm. The circumference is π × 631.9 = 1,985.2 mm, which means the wheel turns 1,000,000 ÷ 1,985.2 = 504 times per kilometre. Switching to 225/45R17 gives 634.3 mm, only about 0.4% larger.

Try this example →

Common mistakes

  • Entering the sidewall height instead of the aspect ratio. The middle number of the code is a percentage of the width, not a size in mm.
  • Entering the rim diameter in millimetres. The field is inches, so 16, not 406.
  • Comparing only the rim size when changing wheels. A 17-inch wheel with a lower profile can match the original overall diameter, which is the figure to compare.
  • Expecting the speedometer error to be shown. The tool gives the diameter of each size, and the difference between them is the percentage error.
  • Assuming a size that matches mathematically will fit. Width, offset and load rating are not checked.

About this calculator

This calculator works out a tire's sidewall (aspect) height, overall diameter, rolling circumference, and revolutions per kilometer from its size code — the width, aspect ratio, and rim diameter printed on the sidewall (like 205/55R16) — using the standard formulas tire shops and enthusiasts use to translate that code into real dimensions.

Drivers considering a "plus-size" fitment (wider tires, smaller sidewalls) use it to keep the overall diameter close to stock so the speedometer stays accurate, and mechanics and tire shops use it to compare alternate sizes before recommending a swap.

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Comments (1)

  • Editorial team

    Overall diameter is the number to compare between two sizes, because the car covers distance in proportion to it. A tire that is 1 percent taller carries you about 1 percent farther per wheel turn, so with the speedometer showing 100 you are really doing about 101. The revolutions per kilometre line tells you the same thing from the other side. Which swap are you weighing, and what size is on your door placard?

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