Decode tire size notation

Tire sidewalls compress several important dimensions into a short code that is easy to copy but not always easy to understand. This decoder turns a standard notation such as 225/45R17 into clearly labeled values for section width, aspect ratio, construction method, and rim diameter. It also normalizes letter case and rejects incomplete or malformed codes, giving drivers, retailers, mechanics, and software systems a dependable interpretation without guessing what an unusual string was meant to say.

Read each part of a tire size in order

A standard code such as 225/45R17 contains four connected facts. The first number is the nominal section width in millimeters, measured across the widest part of an appropriately mounted tire rather than across its tread alone. The number after the slash is the aspect ratio: the sidewall height expressed as a percentage of that section width.

It is not a height in millimeters. The following letter identifies construction. R means radial, D means diagonal, and B means bias-belted.

The final number is the wheel or rim diameter in inches. This mixed use of metric width and imperial rim diameter is normal industry notation, not a conversion error. The decoder separates these fields and returns explicit unit-bearing names so a value cannot be mistaken for a different measurement.

It also returns a normalized notation, which is useful when an operator entered a supported construction letter in lowercase or surrounded an otherwise valid code with harmless whitespace. No estimated dimensions are added to the decoded result.

Understand what the decoded values do and do not guarantee

The decoded section width is nominal, so an actual mounted tire may measure somewhat differently because of its manufacturer, product design, approved rim-width range, inflation, load, and measuring conditions. The aspect ratio describes the relationship between nominal sidewall height and nominal section width. For a 225/45 tire, the nominal sidewall height is 45 percent of 225 millimeters, but this capability deliberately reports the encoded ratio instead of presenting a derived measurement as a certified physical dimension.

The construction field explains the internal arrangement named by the notation. Radial construction is overwhelmingly common on modern passenger vehicles, while diagonal and bias-belted codes remain meaningful for some specialty or older applications. Rim diameter tells you the diameter category of the wheel seat, but matching that number alone does not establish compatibility.

Load index, speed rating, service type, overall diameter, width clearance, wheel width, and vehicle requirements can all matter. Use the result to understand the code, then consult the vehicle and tire manufacturer before selecting or fitting a tire.

Validate clean notation before storing or comparing it

A parser is valuable because tire codes often arrive through forms, inventory feeds, support messages, or copied listings. Small mistakes can silently corrupt comparisons when a system treats the whole code as unverified text. This decoder requires the compact width/aspect-construction-rim structure: three width digits, a slash, two aspect-ratio digits, one supported construction letter, and two rim-diameter digits.

It trims whitespace around the complete value and accepts lowercase R, D, or B, but it does not ignore missing separators, embedded spaces, decimal numbers, extra service prefixes, load indexes, speed symbols, or arbitrary trailing text. Those additions can carry separate meanings and should be modeled separately rather than discarded. A malformed value produces an input error instead of a partial result.

That fail-closed behavior makes the capability suitable for validation at an ingestion boundary. Store the normalized code beside the separated values, retain any additional tire markings in dedicated fields, and route rejected records for correction.