A code such as 205/55 R16 combines measurements that use different units. The width is nominal millimetres, the profile is a percentage, and the rim code is inches. Reading every position separately prevents a common mistake: treating 55 as millimetres or 16 as the tire’s outside diameter. The code describes nominal dimensions, not a complete replacement approval.
205 is nominal section width
The first number represents nominal section width from sidewall to sidewall in millimetres under a defined measuring condition. TireCalc uses 205 mm exactly in the formula. It does not claim that every mounted 205 tire will measure exactly 205 mm because rim width, construction, pressure, load, and manufacturing tolerance affect a physical measurement.
55 is a ratio, not a height
The aspect ratio says one sidewall is 55 percent of the nominal width. For 205/55, sidewall height is 205 × 55 ÷ 100 = 112.75 mm. Two sidewalls contribute to the outside diameter. Keeping the percentage as an exact fraction avoids rounding before the rim diameter is added.
R identifies radial construction; 16 is the rim code
R describes radial construction. The following 16 is the nominal rim diameter in inches, converted with the exact definition 1 inch = 25.4 mm. Other sidewall markings can describe load index, speed symbol, reinforced construction, severe-snow qualification, date, or certification. TireCalc deliberately does not parse or validate them, so its geometric result cannot replace the vehicle placard, manual, manufacturer, or tire professional.