Decode any tire size.
See every dimension.
Enter a size like 215/55R17 for the full breakdown — section width, sidewall height, overall diameter in inches, circumference, and revs per mile. Compare two sizes to see the exact speedometer error.
Three tools. All free. All instant.
Tire size decoder
Section width, sidewall height, overall diameter in inches and mm, circumference, and revs per mile — all from a single size string.
Decode a size →
Compare two sizes
Diameter difference, speedometer error at 60 mph with a visual gauge, odometer effect, ground clearance change, and fitment verdict.
Compare sizes →
Plus sizing
Enter any base size and see safe plus-0, +1, and +2 rim options with diameter deltas — all within the 3% rule.
Explore plus sizing →Pure math. No database fees. No upsells.
Every dimension is calculated from standard industry formulas (ETRTO / TRA). There is no database behind the scenes, no API call, no black box. All math runs in your browser — you can verify it yourself. We do not show you a result and then ask you to subscribe to see the details.
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Frequently asked questions
What does a tire size like 215/55R17 mean?
215 is the section width in millimeters, 55 is the aspect ratio (sidewall height as a percentage of width), and 17 is the rim diameter in inches. Decode any size instantly with the tool above.
Is the calculator really free?
Yes — completely free, no account, no paywall. All calculations run in your browser using standard tire-industry formulas (ETRTO/TRA). No data leaves your device.
What is revolutions per mile?
Revolutions per mile (RPM) is how many times a tire rotates per mile of travel. Your speedometer and odometer are calibrated to this number. A larger tire has fewer revolutions per mile, making the speedo read low.
How much diameter difference is safe between tire sizes?
The widely-accepted rule is a maximum 3% overall diameter change. Up to 5% is sometimes done but requires checking wheel-well clearance and ABS calibration. Beyond 5% is generally problematic.
What is plus sizing?
Plus sizing means going up one (or two) rim diameter inches while dropping the aspect ratio to keep the overall tire diameter nearly the same. Plus-1 = +1 rim inch, minus roughly 10 AR points. Use the Plus Sizing tool to see safe options for your size.
Fitment disclaimer: Tire size math is deterministic, but fitment depends on your specific vehicle. Always confirm with a qualified tire professional and your vehicle's door-placard before purchasing. Incorrect tire sizes can affect speedometer accuracy, ABS/traction-control calibration, and load capacity.