Formula
- Use the units shown beside each field.
- Results update automatically whenever you change an input.
- Displayed values use up to four decimal places, or scientific notation for very small or large numbers. Calculations use standard floating-point arithmetic; exact integer and fraction tools identify their own precision rules.
- Unloaded nominal geometry. Actual rolling circumference, pressure, tire growth, fitment and speedometer calibration can differ.
Example Calculation
- Original width: 205 mm
- Original aspect ratio: 55 %
- Original rim: 16 in
- New width: 225 mm
- New aspect ratio: 45 %
- New rim: 17 in
205/55R16 is nominally 631.9 mm; 225/45R17 is 634.3 mm, about 0.38% larger.
Frequently asked questions
How do I use this calculator?
Enter original width, original aspect ratio, original rim, new width, new aspect ratio, new rim. The result updates automatically. Use Reset to restore the example values.
What method does it use?
diameter=rim inches × 25.4+2 width × aspect/100; ratio=new/original.. Unloaded nominal geometry. Actual rolling circumference, pressure, tire growth, fitment and speedometer calibration can differ.
Why might a rounded result differ?
The calculation keeps full numeric precision internally, then rounds the displayed result. Rounding intermediate steps by hand can produce a slightly different answer.
Are my inputs saved online?
No. This calculator processes your inputs in your browser. A recently used list stores only calculator names on this device, not your entered values.
Check the assumptions and units before using this result. Read about calculation methods.