Tire size calculator
Overall diameter, sidewall height and revolutions per kilometre from a tire code.
Fill in the fields and the result will appear here automatically.
Reads a code like 205/55 R16, whose three numbers are written in different units — and that is the whole calculation. The width is in millimetres, the profile is a percentage of that width, and the rim diameter is in inches. The middle figure is therefore not a height: 55 on a 205-wide tire means 112.75 mm, and reading it as millimetres is out by a factor of two. The overall diameter is the rim plus two sidewalls, top and bottom, and from it follow the circumference and the revolutions per kilometre used to compare sizes and estimate speedometer error.
How it works
Formula and logic
Sidewall = width × profile ÷ 100. Overall diameter = rim diameter × 25.4 + two sidewalls. Circumference = π × overall diameter, and revolutions per kilometre = a million divided by it. This is nominal unloaded geometry from the marking. Actual rolling diameter and loaded circumference depend on construction, pressure, load and wear and are not measured here. All three inputs are positive. The calculation does not verify vehicle approval, clearance, load index or speed rating.
Example
A 205/55 R16 tire has a 112.75 mm sidewall and a 631.9 mm overall diameter — 503.73 revolutions per kilometre.
Fields and units
- Tire width, mm — mm
- Profile, % of width — %
- Rim diameter, inches — in
How to use
- — Enter the first number of the code — the tire width in millimetres.
- — Enter the second number — the profile as a percentage of the width, not in millimetres.
- — Enter the rim diameter in inches, the figure after the R.
- — Compare the overall diameter against another size.
Method and limitations
- Calculation method
- Formula and logic
- Data or methodology source
- Michelin: nominal width, aspect percentage and rim diameter in tyre markings NIST SP 811 B8: units and metric PS versus mechanical hp; tabulated factors are rounded
- Limitation
- This is nominal unloaded geometry from the marking. The calculation does not verify vehicle approval, clearance, load index or speed rating.
FAQ
Why is the middle figure not a height in millimetres?
Because it is a percentage of the width. On a 205/55 tire the sidewall is 55% of 205, that is 112.75 mm rather than 55 mm.
Why does the sidewall count twice?
The overall diameter runs through the centre of the wheel, and there is sidewall both below and above the rim. Two sidewall heights are therefore added to the rim diameter.
What are revolutions per kilometre for?
They compare sizes: if a new tire turns fewer times per kilometre, the speedometer will start reading low and so will the odometer.
How do I estimate speedometer error?
With unchanged calibration, the geometric speed scale changes by Dnew/Dold. A larger diameter means more distance per turn. This compares nominal sizes rather than measuring a particular speedometer’s error.
Is deflection under load included?
No. The result uses the marking and nominal circular geometry. Actual rolling requires manufacturer data or measurement at a specified load and pressure; there is no universal few-millimetre correction here.