Enter a tire’s width, aspect ratio and rim size to get its overall diameter, circumference and revolutions per mile.
How the tire size calculator works
Tire codes like 225/45R17 encode the size. Enter the three numbers and this calculator returns the sidewall height, overall diameter, circumference and how many times it turns per mile.
Useful when comparing tire sizes or checking speedometer effects.
Worked example
On the values this calculator opens with, the overall diameter is 24.97″. Underneath, Sidewall comes out at 3.99″ and Circumference at 78.5″. Change any field and every figure updates as you type.
Getting your answer
- Enter the width in millimetres.
- Enter the aspect ratio.
- Enter the rim diameter in inches.
The maths behind it
sidewall = width × aspect% · diameter = rim + 2 × sidewall
revs per mile = 63360 ÷ circumference (in)
FAQ
What do the numbers in a tire size mean?
In 225/45R17: 225 is width in mm, 45 is the sidewall as a percent of width, and 17 is the rim diameter in inches.
Why does tire diameter matter?
A different overall diameter changes your speedometer reading and gearing. Keep replacements close to the original.
What are revolutions per mile?
How many times the tire turns to cover a mile — smaller tires turn more often.