Tire Size CalculatorFree online other calculator — instant results, no sign-up
Diameter and circumference from a tire code.
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About the Tire Size Calculator
This calculator computes the total diameter, outer circumference, and revolutions per kilometer using standard metric tire sizing specifications. It is commonly used by drivers, mechanics, and automotive enthusiasts to compare tire dimensions when upgrading wheels or verifying speedometer accuracy.
- Width
- Nominal tire section width in millimeters, typically between 125 and 355 mm.
- Aspect ratio
- Sidewall height expressed as a percentage of the section width, typically between 20 and 85.
- Rim
- Wheel rim diameter in inches, commonly between 13 and 24 inches.
Formula and method
d = (r × 25.4) + (2 × w × a / 100) Circumference = π × d Revs per km = 1,000,000 / Circumference
- wTire width in millimeters
- aAspect ratio (sidewall height as a percentage of width)
- rWheel rim diameter in inches
- dTotal tire diameter in millimeters
The tool converts rim diameter from inches to millimeters using 25.4 and adds the height of two sidewalls to determine the total diameter. Circumference is calculated using π times diameter, and revolutions per kilometer are derived by dividing 1,000,000 millimeters by circumference. Diameters are rounded to one decimal place (mm) and two decimal places (inches), while circumference and revolutions are rounded to the nearest integer.
Worked example
These are the calculator's starting values and the results it actually produces for them.
Inputs
- Width225 mm
- Aspect ratio45
- Rim17 in
Results
- Diameter634.3 mm (24.97 in)
- Circumference1,993 mm
- Revs per km502
How to use the Tire Size Calculator
- 1Multiply the tire width by the aspect ratio divided by 100 to get a single sidewall height in millimeters.
- 2Multiply the rim diameter in inches by 25.4 to convert wheel diameter to millimeters.
- 3Add twice the sidewall height to the rim diameter to get the total tire diameter.
- 4Multiply total diameter by π to calculate circumference, then divide 1,000,000 mm by that circumference to obtain revolutions per kilometer.
Reading your result
Diameter shows the overall standing height of the tire in both millimeters and inches. Circumference indicates the distance traveled during a single complete rotation of the wheel. Revolutions per kilometer shows the rolling frequency, which is useful for checking whether an alternate tire size will skew the vehicle's speedometer or odometer.
Limitations
Calculations represent theoretical dimensions based on nominal tire specifications. Actual mounted tire dimensions will vary due to rim width, inflation pressure, vehicle load deflection (rolling radius), and tread wear.
Common questions
What do the numbers on a tire size like 225/45R17 mean?+
The first number (225) is the tire width in millimeters, the second (45) is the aspect ratio indicating sidewall height is 45% of the width, and the final number (17) is the rim diameter in inches.
Why do revolutions per kilometer matter?+
Vehicle speedometers and odometers rely on wheel revolutions to estimate speed and distance. A substantial change in revolutions per kilometer between tire sizes causes speedometer inaccuracies.
Does this calculate loaded rolling radius?+
No, it calculates the static, unloaded outer diameter of the tire based strictly on nominal manufacturer dimensions.
Does the calculator save my information?+
Calculations run in your browser. While signed in, inputs and results are automatically recorded in account history; saving a result creates a separate saved record.
Further reading
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Results are estimates for informational purposes and are not financial, medical or legal advice.