Tire Size Calculator

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.

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.