mm
%
Result
Overall diameter599.8 mm
Circumference (unloaded)1,884 mm
How it is calculated, with your numbers
- Sidewall height
side = w × ar / 100120 × 70 / 100 = 84.0 mm - Overall diameter
dia = rim × 25.4 + 2 × side17 × 25.4 + 2 × 84 = 599.8 mm - Circumference (unloaded)
circ = π × (rim × 25.4 + 2 × side)π × (17 × 25.4 + 2 × 84) = 1,884 mm
Working uses the metric units the formula is written in.
- w Section width (mm)
- ar Aspect ratio (%)
- rim Rim diameter (inches)
- side Sidewall height
- dia Overall diameter
- circ Circumference (unloaded)
What it means
In a size like 120/70-17, 120 is the nominal section width in millimetres, 70 is the sidewall height as a percentage of that width, and 17 is the rim diameter in inches. Two sidewalls plus the rim give the overall diameter.
These are nominal figures. Real tyres of the same size differ by a few millimetres between makes and models, and a loaded tyre rolls on a slightly smaller effective radius.
Assumptions and limits
- Nominal dimensions per the size marking.
- Inch-size tyres (e.g. 3.00-21) are not covered by this formula.
Sources
- ETRTO Standards Manual: tyre size designations (width / aspect ratio / rim diameter).
Related calculators
Speedometer error after a tyre size changeHow far off the speedometer reads with a different tyre size.Rake and trail calculatorGround trail from steering head angle, fork offset and front wheel size.Road speed in gear calculatorSpeed at any rpm from primary, gear and final drive ratios and tyre size.