Decode any tire size.
See every dimension.
Type a size like 215/55R17 for the full breakdown. Then put a replacement beside it: visual overlay, speedometer error, ground-clearance change, and the fitment risks the math alone won't catch.
Fitment starts with the OE size, then the math.
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, visual tire overlay, odometer effect, ground clearance change, and fitment risk notes.
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Vehicle OE size entry
Start from the door-placard original size, then compare replacement options against the factory baseline and the ±3% rule.
Start from OE size →Pure math. No database fees. No upsells.
Every dimension comes from standard industry formulas (ETRTO / TRA). No database behind the scenes, no API call, no black box. The math runs in your browser, so you can check it yourself. We never show you a result and then hide the details behind a sign-up.
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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.
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