Tire size calculator

Read metric tire size as width, sidewall percentage and rim inches, then compare nominal outer diameters.

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Tire width (mm)

Sidewall aspect ratio (%)

More inputs and assumptions

Rim diameter (inches)

Comparison width (mm)

Comparison aspect ratio (%)

Comparison rim (inches)

Answer

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Read metric tire size as width, sidewall percentage and rim inches, then compare nominal outer diameters.

More results

Precision and calculation record

Decimal places in numeric results and text downloads

Rounding changes displayed numbers, not the calculation. Text, fractions, matrices and CSV schedules keep their own formatting. Automatic keeps up to 12 significant digits, with money shown to cents where applicable.

Saved scenario

Save the current answer, then change an input to compare. This clears when you leave.

Formula and assumptions

Diameter = two sidewalls + rim inches × 25.4; sidewall = width × aspect ratio/100. These are unloaded nominal dimensions, not rolling circumference or fitment approval. Rim width, load/speed rating, clearances and actual manufacturer dimensions still matter.

Automatic precision shows numbers to 12 significant digits. Thousands separators such as 1,000 are accepted; use a dot for decimals.

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Updated 2026-09-26

Using Tire size calculator

Read metric tire size as width, sidewall percentage and rim inches, then compare nominal outer diameters.

Next: Speedometer error calculator

A worked example

Example inputs: tire width (mm) = 205, sidewall aspect ratio (%) = 55, rim diameter (inches) = 16, comparison width (mm) = 225, comparison aspect ratio (%) = 45, comparison rim (inches) = 17. Nominal outer diameter (mm) = 631.9.

Before you use the result

Example results for tire width (mm)

These examples use Sidewall aspect ratio (%): 55; Rim diameter (inches): 16; Comparison width (mm): 225; Comparison aspect ratio (%): 45; Comparison rim (inches): 17. They are reference calculations, not recommended settings.

Tire width (mm)	Nominal outer diameter (mm)
102.5	519.15
205	631.9
410	857.4

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