Tire Size Calculator
Calculate tire diameter, circumference, sidewall height, and compare alternate sizes.
Calculate a tire's overall diameter, circumference, sidewall height, and revolutions per mile from its metric size code, and compare it against an alternate tire size.
How Tire Size Calculator Works
A tire size like 225/45R17 encodes three measurements: section width in millimeters (225), aspect ratio as a percentage of that width (45%), and rim diameter in inches (17). The calculator first finds sidewall height in inches: width × (aspect ÷ 100) ÷ 25.4 (converting millimeters to inches).
Overall diameter is the rim size plus two sidewall heights (one on each side of the wheel): diameter = rim + 2 × sidewall. Circumference is then diameter × π, and revolutions per mile is 63,360 (inches in a mile) divided by that circumference.
If you enter a second, "compare" tire size, the calculator works out the same figures for it and reports the percentage difference in overall diameter — useful for checking whether an alternate tire size will throw off your speedometer or ground clearance.
See It In Action
Who Uses Tire Size Calculator and Why
- Checking whether an alternate tire size will fit without throwing off speedometer accuracy before buying new tires.
- Comparing a plus-sized wheel-and-tire combo against the factory size to see how much the overall diameter would change.
- Working out a tire's sidewall height and overall diameter from its size code alone, without needing to measure a physical tire.
- Confirming that a replacement tire size close to, but not exactly matching, the original will stay within a safe diameter tolerance.
Mistakes to Avoid
- Assuming a bigger rim size alone means a bigger tire — increasing rim diameter while keeping overall tire diameter constant (a common goal when upsizing wheels) actually requires a lower-profile tire with a shorter sidewall, not a taller one.
- Comparing two tire sizes with different width and aspect ratio changes at once and assuming the diameter difference is intuitive — the interaction between width, aspect ratio, and rim size isn't always obvious without running the numbers.
- Ignoring a diameter difference beyond roughly 3% and assuming it won't matter — larger mismatches can meaningfully throw off speedometer readings and affect ABS or traction control calibration.
Tips for Best Results
- When comparing an alternate size, aim to keep the overall diameter difference within about 3% of the original to avoid speedometer error and driveline stress.
- Remember the three numbers in a size like 225/45R17 are width in millimeters, aspect ratio as a percentage, and rim diameter in inches — enter each value in the field matching that unit, not the raw digits from the tire code interchangeably.
Fixing Common Problems
The comparison shows a large percentage difference I wasn't expecting. — Double check that width, aspect ratio, and rim size were entered correctly for both the original and comparison tire — a single digit typo in aspect ratio or rim size can produce a much larger diameter swing than intended.
I'm not sure what aspect ratio to enter. — It's the middle number in a tire size code like 225/45R17 — in this example, 45 — representing sidewall height as a percentage of the tire's width, not an absolute measurement.
Terms Explained
Aspect ratio: The tire's sidewall height expressed as a percentage of its width — for example, an aspect ratio of 45 means the sidewall is 45% as tall as the tire is wide.
Plus sizing: Increasing wheel diameter while lowering tire aspect ratio to keep overall tire diameter close to the original, a common approach when upgrading to larger wheels.