Decode a tire size like 205/55R16 into width, sidewall height, overall diameter and circumference.
A tire size like 205/55R16 packs three numbers into a compact code that mixes metric and imperial units and a percentage. The first number (205) is the tread width in millimetres, the second (55) is the sidewall height as a percentage of the width — called the aspect ratio or profile — and the third (16) is the rim diameter in inches. The R means radial construction. This tool decodes all of that into plain numbers: the sidewall height in millimetres, the overall diameter from the ground to the top of the tire, the circumference and the number of revolutions per kilometre.
The overall diameter matters more than any single number because it is what determines speedometer accuracy, ground clearance and whether the tire fits the wheel well. Two tires with different markings can have the same diameter: a 195/65R15 and a 205/55R16 are within a few millimetres of each other. The circumference is what the speedometer counts — a tire that is 3% larger in circumference makes the speedometer read 3% low, which is enough to matter for speed cameras and fuel consumption calculations.
Everything here is arithmetic from the three numbers on the sidewall. The calculation assumes a new tire at its nominal dimensions; wear, inflation pressure and load all change the real numbers slightly. The rim diameter is converted from inches to millimetres (× 25.4), the sidewall height is the width times the profile divided by 100, and the overall diameter is the rim plus twice the sidewall. No lookup table, no database, no network request.
Sidewall = width × profile / 100. Overall diameter = (rim × 25.4) + (2 × sidewall). Circumference = π × diameter. Revolutions per km = 1,000,000 / circumference.
Radial construction — the internal cords run radially from bead to bead. Almost every passenger tire sold today is radial. A D or B would mean diagonal (bias-ply) or belted bias, which are rare outside specialty or vintage applications.
The Z (or W, Y) is a speed rating and does not change the dimensions. Ignore it and type 225/45R17.
Yes — type one, note the diameter and circumference, then type the other. If the diameters are within 3% of each other the swap is generally safe; outside that range the speedometer error and clearance risk grow.
Historical convention. When tire widths were standardised, the profile was defined relative to the width so that a 55-series tire on any width has the same proportions. A lower number means a shorter, stiffer sidewall.
Yes, slightly. An underinflated tire bulges at the sidewall and reduces the effective diameter by a few millimetres. The numbers here assume the nominal dimensions at the recommended pressure.
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