Tire Size Calculator

How to Read a Tire Size Code

A tire size code like 225/45R17 packs three separate measurements into one string. The first number (225) is the tire's width in millimeters, measured sidewall to sidewall. The second number (45) is the aspect ratio — the sidewall height expressed as a percentage of that width, so a lower number means a shorter, lower-profile sidewall. The letter R means radial construction, the standard for nearly all modern tires. The final number (17) is the wheel diameter in inches that the tire is built to mount on.

Overall diameter, circumference, and revolutions per mile matter beyond just knowing the code — they affect speedometer accuracy after a tire size change, whether a spare tire is a genuinely safe match for the tires already on the vehicle, and gearing calculations for anyone modifying a drivetrain.

Worked Example

For 225/45R17: sidewall height = 225 × 0.45 = 101.25mm, which converts to 101.25 ÷ 25.4 = 3.99 inches. Overall diameter = wheel diameter + 2 × sidewall height = 17 + 2 × 3.99 = 24.98 inches. Circumference = π × 24.98 ≈ 78.5 inches. Revolutions per mile = 63,360 ÷ 78.5 ≈ 807 revolutions for every mile driven.

A Brief History of the Tire Size Code

Tire sizing wasn't always this standardized. Through the mid-20th century, US tires carried numeric or alphanumeric codes that described a tire's rough size and load capacity, but weren't consistent enough for reliable cross-brand comparison. European manufacturers moved to a fully metric sizing system in the 1960s, standardized through their own regional tire and rim organization, producing sizes with no letter prefix at all — commonly called Euro-metric or ISO-metric sizes today.

The US industry adopted its own metric-based system in the mid-1970s, called P-metric ("P" for "passenger"), standardized by the Tire and Rim Association. P-metric blended the two conventions already in use worldwide: section width in millimeters, matching the European approach, but wheel diameter in inches, matching the older US convention — the same hybrid format this calculator reads today. Both P-metric and Euro-metric/ISO-metric codes remain in wide use, which is why two visually similar tire codes can occasionally follow slightly different underlying standards.

Common Mistakes

Confusing overall diameter with wheel diameter is the most common mix-up — they're not the same thing. Wheel diameter is only the size of the metal rim, while overall diameter also includes the sidewall height stacked on both sides of that rim. A second common mistake is assuming two tire codes are interchangeable just because they share the same wheel diameter; if their overall diameters differ enough, the swap can throw off speedometer accuracy even though both tires technically fit the same wheel.

Terms You Should Know

Aspect Ratio — the tire's sidewall height expressed as a percentage of its width; a lower aspect ratio means a shorter, lower-profile sidewall.

Sidewall Height — the vertical distance from the wheel rim to the outer edge of the tread, calculated from width and aspect ratio.

Radial Construction — the "R" in a tire size code; indicates the tire's internal fabric plies run radially (perpendicular to the direction of travel), the standard construction method for nearly all tires sold today.