Car & Vehicle

Tyre Size Calculator

Read a tyre size in millimetres: sidewall height, overall diameter and rolling circumference.

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Your figures

As printed on the sidewall - 205/55 R16, for example. Width in millimetres, sidewall as a percentage of the width, rim in inches.

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Result

Your result

Enter your figures and the result appears here.

Estimates only. Rates, fees and specifications change. Confirm against the official source before you rely on a figure — see our disclaimer.

About this calculator

A size like 205/55 R16 packs three units and two measuring systems into seven characters. The first number is the width in millimetres. The second is not a measurement at all — it is the sidewall height as a percentage of that width. The last is the rim diameter in inches.

This page reads the size and gives you the dimensions it implies: how tall the sidewall is, how big the whole wheel is, and how far it travels in one turn.

How to use this calculator

  1. Read the size off the sidewall of the tyre.
  2. Type it in. 205/55 R16, 205/55R16 and 205 55 16 are all accepted.

Anything that is not a tyre size is refused rather than guessed at. The page will tell you what the format looks like.

The formula

Sidewall      = Width x Aspect ratio / 100
Diameter      = 2 x Sidewall + Rim x 25.4
Circumference = pi x Diameter
Turns per km  = 1,000,000 / Circumference in mm

The sidewall is counted twice because there is one above the rim and one below it. The rim is converted from inches at exactly 25.4 mm, which is the definition of the inch rather than a measurement of it.

SymbolMeaningUnit
Width Section width mm
Aspect ratio Sidewall as % of the width %
Rim Wheel diameter inches

Worked example

205/55 R16:

Sidewall      = 205 x 0.55        = 112.8 mm
Diameter      = 2 x 112.8 + 406.4 = 631.9 mm
Circumference = pi x 631.9        = 1,985.2 mm
Turns per km  = 1,000,000 / 1,985.2 = 504

Notes and limits

  • These are dimensions, not a fitment check. Whether a tyre suits a vehicle depends on the wheel arch, the suspension, the load and speed ratings and the manufacturer's own specification — none of which is contained in a size.
  • The correct sizes for your vehicle are on the placard in the driver's door frame and in the handbook.
  • The letter in the middle is the construction. R is radial, which is what almost every modern car tyre is; D is diagonal and B is bias belted.
  • Numbers after the size — 91V, for instance — are the load index and speed rating. They matter for fitment and are not part of the dimensional arithmetic.
  • A mounted tyre's real diameter differs a little from the calculated one with pressure, load and wear.

What this calculation assumes

  • Dimensions are nominal, calculated from the size rather than measured on a mounted tyre.
  • This is not a fitment check and does not say whether a tyre suits any vehicle.

Frequently asked questions

What do the numbers on a tyre mean?

In 205/55 R16: 205 is the section width in millimetres, 55 is the sidewall height as a percentage of that width, R means radial construction, and 16 is the rim diameter in inches.

How do I work out a tyre's overall diameter?

Multiply the width by the aspect ratio to get the sidewall, double it because there is a sidewall above and below, and add the rim diameter converted at 25.4 mm per inch. For 205/55 R16 that is 631.9 mm.

Will this tyre fit my car?

This page cannot tell you. A size is a dimension, not a fitment - clearance, load rating, speed rating and the manufacturer's specification all matter and none of them is in the size. Check the placard in the door frame.

What is the aspect ratio?

The sidewall height expressed as a percentage of the section width. A 55 on a 205-wide tyre means a sidewall of 112.75 mm. It is why the same aspect ratio gives a taller sidewall on a wider tyre.

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