Tire Diameter
Calculator
Convert a metric tire designation into nominal unloaded diameter, sidewall height and circumference. Actual rolling dimensions depend on tire construction and load.
Your setup
Your results
CalculatedThe aspect ratio is sidewall height as a percentage of nominal section width.
Nominal unloaded geometry only. Loaded radius, pressure, tread wear, construction and growth affect actual rolling circumference.
How the calculation works
Follow the formula and keep the input units consistent at every step.
sidewall(mm) = width × aspect/100; diameter(mm) = rim(in) × 25.4 + 2 × sidewall- Read the width, aspect and rim diameter from a designation such as 205/55R16.
- Multiply width by aspect fraction for one sidewall.
- Add two sidewalls to the converted rim diameter.
A Worked Example
205/55R16: one sidewall is 112.75 mm. Diameter is 406.4 + 225.5 = 631.9 mm (24.88 in); geometric circumference is approximately 1,985.22 mm.
What Your Result Means
The aspect ratio is sidewall height as a percentage of nominal section width.
Assumptions & Limitations
Nominal unloaded geometry only. Loaded radius, pressure, tread wear, construction and growth affect actual rolling circumference.
Common Mistakes
The 55 in 205/55R16 is a percentage, not a millimeter measurement. Tire designations retain mm and inches in both unit modes.
Questions about this calculation
Which values should I enter?
Read the width, aspect and rim diameter from a designation such as 205/55R16. The 55 in 205/55R16 is a percentage, not a millimeter measurement. Tire designations retain mm and inches in both unit modes.
How should I interpret the result’s precision?
Displayed results are rounded for readability. Intermediate calculations retain floating-point precision; extra displayed decimals do not improve your input accuracy.
Can I use metric units?
Yes. Select Metric above the calculator to convert your inputs. Results always state their units. Tire-size designations keep their conventional mm and inch units.
What are the main limitations?
Nominal unloaded geometry only. Loaded radius, pressure, tread wear, construction and growth affect actual rolling circumference.
Method, sources & relevant guides
Angular speed and tangential speed. Gear reductions follow tooth engagement and conservation of rotation count.
OpenStax — University Physics: rotation with constant angular accelerationMethod documentation updated October 18, 2026. Independent professional review has not been claimed. Read the calculation and review policy.