Calculate tire diameter, circumference, sidewall height, and revolutions per km/mile in both metric and imperial units.
The tire circumference calculator is a fundamental engineering tool that precisely measures and calculates all factors determining a tire's geometric shape. Circumference—the actual distance a tire travels in one revolution—is the most critical metric as it serves as the reference point for all vehicle driving data, including speed, distance, and RPM. This tool goes beyond simple arithmetic to provide deep insights into a tire's physical volume and surface area.
Mathematically, it multiplies the tire diameter by pi (approx. 3.14159) while handling both metric and imperial units for global compatibility. It also derives sidewall height from the relationship between section width and aspect ratio, simulating the actual mounted outer diameter including the rim flange. Data on revolutions per kilometer or mile is provided as a foundational resource for predicting tire tread wear rates and overall lifespan.
Enter tire size (width/aspect ratio/rim diameter) to calculate diameter, circumference, sidewall height, and revolutions per km.
Sidewall height = Width × (Aspect Ratio / 100). Tire diameter = (Rim diameter × 25.4) + (Sidewall height × 2)
Results are shown in both metric and imperial units. Use this to compare tire sizes before purchasing.
Diameter is built from two parts: rim × 25.4 (inches to mm) plus twice the sidewall height, where sidewall = width × aspect/100. Worked example for 245/40R18: sidewall = 245 × 0.40 = 98 mm, so diameter = 18 × 25.4 + 2 × 98 = 457.2 + 196 = 653.2 mm, and circumference = 653.2 × π ≈ 2,052 mm. Each full revolution therefore covers about 2.05 m.
Revolutions per kilometer follow 1,000,000 / circumferenceMm; the 245/40R18 above turns about 487 revs/km (≈784 revs/mile). A frequent mistake is treating the rim diameter as the tire diameter — the rim is only the wheel, and the two sidewalls add the rest. Aspect ratio is a percentage of the section width, not an absolute number, so widening the tire while keeping the aspect ratio also raises the sidewall and the overall diameter.
These geometric values use the unloaded nominal diameter defined by ISO/ETRTO; published 'revs per mile' from manufacturers reflect the loaded radius and run a few percent higher because the contact patch flattens the bottom of the tire. Inflation pressure, load, and even speed (centrifugal growth at high RPM) all shift the effective rolling circumference away from the static calculation.