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Tire Size Calculator

Decode any tire size code (e.g. 225/45R18) to get overall diameter, sidewall height, rolling circumference, and revolutions per mile.

The three-digit number at the start of the tire code, e.g. 225 in 225/45R18
The two-digit percentage after the slash, e.g. 45 in 225/45R18
The number after the R, e.g. 18 in 225/45R18

Results

Overall Diameter25.97"
Sidewall Height101.3 mm
Rolling Circumference81.59"
Revolutions per Mile777 rev/mile

📖What is it?

Tire size codes like 225/45R18 encode three measurements: section width in mm, aspect ratio (sidewall height as % of width), and rim diameter in inches. This calculator decodes those values into real-world dimensions so you can compare fitments, check speedometer accuracy, and plan plus-sizing upgrades.

🎯How to use

Read the three numbers from your tire sidewall and enter each into the corresponding field. The results update instantly. To compare two tire sizes, note the overall diameter — sizes within ~3% of each other are generally considered speedometer-safe.

💡Example scenario

A stock tire of 225/45R18 has a 101.25 mm sidewall, giving an overall diameter of 25.9 inches and 804 rev/mile. Upgrading to 235/40R18 yields a 94 mm sidewall and 25.4 inches — roughly 2% smaller, causing a speedometer to read ~2% high.

🏆Pro tip

When plus-sizing (larger rim, lower profile), keep overall diameter within ±3% of stock to avoid ABS/traction-control calibration errors and clearance issues. Many enthusiasts go +1" rim with −10 aspect ratio points to maintain nearly identical rolling diameter.