How to read a metric tire size

Read 225/45R18 in three parts

225 is a nominal section width of 225 mm. The next number, 45, is a percentage: the sidewall height is 45% of that width. R identifies radial construction, and 18 is the rim diameter in inches. The marking deliberately combines metric width with an inch-based rim size. Do not read the final number as the tire’s overall diameter.

Turn the percentage into a length

Multiply 225 by 0.45 to get a nominal sidewall height of 101.25 mm. There is a sidewall above the rim and another below it in a diameter cross-section. The 18-inch rim contributes 457.2 mm, so the total is 457.2 + 2 × 101.25 = 659.7 mm. This is why the calculator asks for all three values.

The same aspect ratio does not mean the same height

Compare 225/45R18 and 245/45R18. Both say 45, but 45% of 245 is 110.25 mm. The second tire’s nominal sidewall is 9 mm taller and its nominal overall diameter is 18 mm larger. A percentage only becomes a physical dimension when you know its base width.

Keep the service description separate

A tire may also carry a prefix, load index, speed symbol or special designation. Those markings carry information that a three-number geometry model cannot replace. Recording only 225/45R18 does not record everything needed to identify an appropriate replacement. Refer to the vehicle’s required specification and the particular tire’s full documentation.

Enter only the geometry you intend to compare

Use the printed-size input when you have a clear metric marking. Check the three parsed fields, especially when copying text from a listing. Manual fields are useful for an explicit dimension experiment; they do not establish that a product with those dimensions exists. The source-backed directory gives a separate reference for selected published markings.

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