Tire Size Converter
Convert tire sizes (width, aspect ratio, rim diameter) to sidewall height and overall diameter in mm and inches.
How to use this tool
- Enter tire width, aspect ratio and rim diameter in the fields above.
- Results update instantly as you type — or click Calculate.
- Read your overall diameter and the full breakdown beneath it.
Enter a tire's width, aspect ratio, and rim diameter to calculate sidewall height and overall diameter.
Formula
Tire sizes are written as Width/AspectRatio R RimDiameter (e.g. 205/55R16).
Sidewall height (mm) = Width (mm) × Aspect Ratio / 100
Overall diameter (mm) = Rim diameter (in) × 25.4 + 2 × Sidewall height
Overall diameter (in) = Overall diameter (mm) / 25.4
How it works
The aspect ratio in a tire designation is a percentage expressing sidewall height as a fraction of tread width; multiplying tread width by aspect ratio / 100 gives the sidewall height. The overall diameter adds the rim diameter (converted from inches to mm at 25.4 mm/in) plus two sidewall heights (one on each side). A common mistake is confusing the rim diameter with the wheel diameter including the tire: the rim diameter in the tire code (e.g. the 16 in 205/55R16) is the bare metal rim only, and the full rolling diameter of the mounted tire will be considerably larger once both sidewalls are added.
Worked example
Calculating overall diameter for a 205/55R16 tire
- Width = 205 mm, aspect ratio = 55%, rim diameter = 16 in.
- Sidewall height: 205 × 55 / 100 = 112.75 mm.
- Rim diameter in mm: 16 × 25.4 = 406.4 mm.
- Overall diameter: 406.4 + 2 × 112.75 = 631.9 mm = 631.9 / 25.4 ≈ 24.88 in.
Sidewall Height: 112.75 mm | Overall Diameter: 631.9 mm (24.88 in)
Common mistakes to avoid
- Reading the aspect ratio as the sidewall height in mm rather than as a percentage of width -- a 205/55R16 tire has a sidewall of 205 x 0.55 = 112.75 mm, not 55 mm.
- Swapping tire sizes with different overall diameters without recalibrating the speedometer -- a larger overall diameter makes the speedometer read lower than actual speed because each wheel revolution covers more ground.
- Ignoring load index and speed rating when selecting replacement tires -- the size dimensions only describe geometry; the load index and speed rating (e.g. 91V) must also meet or exceed the original specification.
Key terms
- Aspect ratio (tire)
- The ratio of sidewall height to tread width, expressed as a percentage; a tire marked 55 has a sidewall height equal to 55% of its tread width.
- Sidewall height
- The distance from the rim edge to the outer tread surface, calculated as tread width × aspect ratio / 100; it affects ride comfort, handling, and load capacity.
- Rim diameter
- The diameter of the bare metal wheel in inches, which is the third number in a metric tire code (e.g. 16 in 205/55R16); it excludes the tire itself.
- Overall diameter
- The full outer diameter of the mounted tire, equal to the rim diameter plus two sidewall heights; it determines speedometer calibration and whether a tire fits without rubbing the wheel arch.
Frequently asked questions
- What does 205/55R16 mean?
- 205 is the tire width in mm, 55 is the aspect ratio (sidewall as % of width), and 16 is the rim diameter in inches. Sidewall height = 205 × 0.55 = 112.75 mm.