Rolling Diameter Calculator
A rolling (loaded) tyre sits slightly squashed, so the distance it covers per revolution is set by its dynamic rolling circumference — smaller than the free-standing geometric circumference.
Enter Values
How to use this calculator
- Enter the tyre's unloaded (geometric) diameter in mm plus a rolling factor to get the effective rolling diameter, circumference and revs per km/mile.
- Alternatively, enter a known rolling circumference or a manufacturer 'revs per km' figure and leave diameter blank — the tool works backwards to the effective rolling diameter and radius.
- Leave the rolling factor blank to use the default 0.97 (about 3% deflection), or set your own between 0.90 and 1.00 (1.00 = geometric, no load deflection).
How it works
A rolling (loaded) tyre sits slightly squashed, so the distance it covers per revolution is set by its dynamic rolling circumference — smaller than the free-standing geometric circumference. The rolling diameter is the unloaded diameter multiplied by a rolling factor (typically 0.96–0.98), and the rolling circumference is π × rolling diameter. The dynamic rolling radius is half the rolling diameter.
From the rolling circumference the tool derives distance-based rev counts: revolutions per km = 1,000,000 mm ÷ circumference (mm), and revolutions per mile = 1,609,344 mm ÷ circumference (mm). Because these all trace back to one circumference, you can supply any one of diameter+factor, circumference, or revs/km and the rest are recovered exactly. Manufacturer 'revolutions per mile/km' data, where published, is the most accurate rolling figure because it already bakes in real deflection.
Worked example
265/70R17 tyre at a 0.97 rolling factor. A 265/70R17 tyre has an unloaded diameter of about 787 mm. Entering 787 mm with a rolling factor of 0.97 gives a rolling diameter of 763.4 mm (30.06 in), a rolling circumference of 2,398.1 mm (2.398 m), a dynamic rolling radius of 381.7 mm, about 417.0 revolutions per km and 671.1 revolutions per mile. That rolling circumference — not the geometric one — is what determines how far the vehicle travels per wheel turn and what the speedo and ABS see.
Common mistakes
- Using the unloaded (geometric) diameter as if it were the rolling diameter — the loaded tyre rolls on a slightly smaller effective diameter, so ignoring the rolling factor overstates distance per turn and revs per km.
- Applying a rolling factor when you already entered a measured rolling circumference or a manufacturer revs figure — those values are already 'loaded', so the tool ignores the factor to avoid double-counting deflection.
- Mixing units: this tool expects diameter and circumference in millimetres. Convert inches to mm (× 25.4) before entering a tyre diameter.
Frequently asked questions
What is rolling diameter and how is it different from tyre diameter?
Tyre (geometric) diameter is the free-standing height of the tyre off the ground. Rolling diameter is the effective diameter the tyre rolls on once it is loaded and slightly deflected — usually a few percent smaller. Because the vehicle travels π × rolling diameter per wheel revolution, rolling diameter (not geometric diameter) is what governs true distance, speedo reading and revs per km.
What rolling factor should I use?
For typical road tyres a factor of about 0.96–0.98 is common; this tool defaults to 0.97. Stiffer or higher-pressure tyres sit nearer 0.98–1.00, while softer off-road tyres at low pressure can be lower. If the manufacturer publishes a 'revolutions per mile' or rolling circumference figure, enter that directly instead — it already accounts for real deflection and needs no factor.
How do you get revolutions per km from the circumference?
One kilometre is 1,000,000 mm, so revolutions per km = 1,000,000 ÷ rolling circumference in mm. For miles it is 1,609,344 ÷ rolling circumference in mm. Enter a revs figure and leave diameter blank to reverse the calculation back to rolling diameter and radius.
Does this account for tyre wear or pressure?
Not directly. Wear reduces effective diameter and under-inflation increases deflection (a lower rolling factor); both shrink the rolling circumference. Adjust the rolling factor, or measure and enter an actual rolling circumference, to reflect your tyre's real condition.
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