Tire Size Calculator
Enter the marking shown on the tire sidewall to calculate the tire’s theoretical physical dimensions.
TIRE SIDEWALL MARKING
Enter the marking
Fields accept numeric text only. You may use a comma or a full stop as the decimal separator, and your entered notation is preserved.
Accepts numbers with an optional decimal separator (comma or full stop).
Accepts numbers with an optional decimal separator (comma or full stop).
Accepts numbers with an optional decimal separator (comma or full stop).
CURRENT MARKING
What this tool does
The Tire Size Calculator converts the marking printed on a tire sidewall into theoretical physical dimensions. A tire size marking is the code printed on the tire, such as 205/55R16. In UK terminology, these are often called tyre sizes; the notation is the same.
The common format: 205/55R16
The format most drivers see on passenger cars, SUVs and many light trucks is the metric format. In 205/55R16, 205 is the nominal section width in millimetres and 55 is the aspect ratio: the theoretical sidewall height expressed as a percentage of that width, not a separate millimetre measurement.
R identifies radial construction and 16 is the wheel rim diameter in inches. A service description may follow, such as 91V, for load index and speed rating. Those values matter when selecting a replacement tire, but they do not change the physical-dimension calculation shown here.
Other tire-size formats it recognises
The calculator also recognises sizes such as 31x10.50R15, where overall diameter comes before width and rim diameter. It accepts metric markings without a stated aspect ratio, such as 185R14.
Two-number agricultural or industrial markings, such as 10-16.5, may state width and rim diameter without enough information to determine overall diameter or sidewall height reliably; supplier technical data remains essential. The tool also accepts bicycle formats such as 32-597, compact imperial formats such as 16x2.125, and selected lawn, garden and small-trailer formats such as 4.80/4.00-8.
Why it matters
Comparing physical dimensions helps show what really changes when two tire sizes are not identical. Circumference is the theoretical distance a wheel covers in one rotation, so a change in overall diameter changes how far the vehicle travels per wheel rotation. If the system has not been recalibrated, the speedometer and odometer calculate from the vehicle's reference tire size: a taller tire causes lower indicated speed and distance, while a shorter tire causes the opposite deviation.
Width and overall diameter are also useful starting points when considering clearance around wheel arches, suspension components and nearby parts. Plus-sizing usually aims to keep overall diameter close while rim diameter increases and aspect ratio decreases.