Calculator
Tire size calculator
The calculator uses nominal width/aspect-ratio and rim-diameter notation. It compares geometry but cannot confirm road approval or physical fitment for a vehicle.
Calculator
Enter data
The width, aspect ratio and rim diameter for the current and proposed tires. Fields marked with an asterisk are required.
How to use the calculator
- Enter the three parameters for the current and proposed tire.
- Review the diameter difference and confirm any replacement against vehicle documentation.
Calculation details
Formula and calculation method
Diameter = 2 × width × aspect ratio ÷ 100 + rim diameter × 25.4 mm. Circumference = diameter × π.
Worked example
A 205/55 R16 tire has a nominal diameter of about 631.9 mm. The calculator compares it with another size and reports the percentage change.
Limitations
- Actual dimensions depend on manufacturer, rim, pressure and wear. Confirm load and speed ratings, offset, clearance and approval.
Sources
Engine version: 1.0 · Last reviewed: 3 August 2026
Frequently asked questions
What diameter difference is acceptable?
There is no universal answer for every vehicle. Confirm the replacement in its approval documents or with a specialist.
Does the calculator check rim width and offset?
No. It compares tire geometry and rim diameter, not complete wheel fitment.