🛞 Tire Size Calculator
A tire code like 205/55R16 packs three numbers: 205 mm tread width, a sidewall 55% of that width, on a 16-inch rim. This calculator combines them into the overall diameter that actually matters for fitment and gearing.
Changing overall diameter changes your speedometer reading and gearing — stay within about 3% of stock size unless you know what you are doing.
How to use this calculator
Enter the three numbers from your tire sidewall, like 205/55R16, to get the overall diameter, circumference and revs per mile.
- Read the size code on your tire sidewall, for example 205/55R16.
- Enter the Tire width (205), Aspect ratio (55) and Rim diameter (16).
- The results appear instantly.
- Read the overall diameter, sidewall height, circumference and revs per mile.
Frequently asked questions
How is the tire diameter calculated?
Sidewall height is width times aspect ratio divided by 100. Overall diameter is the rim diameter plus twice the sidewall, converted to inches.
Can you give a worked example?
A 205/55R16 tire has a 112.75 mm sidewall, a 24.88 inch overall diameter, a 78.16 inch circumference, and turns 811 times per mile.
Why does diameter matter when changing tires?
A different overall diameter changes your speedometer reading, gearing and clearance. Stay within about 3 percent of the stock diameter.
What do the three code numbers mean?
205 is the tread width in millimeters, 55 is the sidewall as a percentage of that width, and 16 is the wheel rim diameter in inches.