How do I convert inches to millimetres?

Multiply inches by 25.4 to get millimetres. This is an exact conversion factor defined internationally. For example, 1 inch = 25.4 mm exactly.

Why do cars use both inches and millimetres?

Wheel diameters are traditionally quoted in inches (14, 15, 16, 17 inch wheels etc.) even in metric countries. However, most mechanical dimensions like bolt sizes, brake disc diameters, and clearances are in millimetres. American parts often use fractional inches.

What are common bolt sizes in both units?

Common metric bolt sizes include M6, M8, M10, M12 (the number is the diameter in mm). Imperial equivalents include 1/4 inch (6.35mm), 5/16 inch (7.94mm), 3/8 inch (9.53mm), and 1/2 inch (12.7mm).

Why does the automotive industry use both inches and millimetres?

British and American cars traditionally used imperial measurements (inches), while European and Japanese cars use metric (millimetres). Modern cars use a mix of both. Wheel sizes are in inches, tyre widths in millimetres, and bolt sizes can be either depending on the manufacturer.

What common automotive parts are measured in inches?

Wheel diameters (15, 16, 17 inch), exhaust pipe diameters, brake disc diameters (some manufacturers), spark plug hex sizes, and American bolt sizes are commonly measured in inches. Most European and Japanese specifications use millimetres.

CarCalcs

Inches to mm Converter

Convert between inches and millimetres for automotive parts and measurements.

Conversion Direction

Conversion direction

Length (inches)

Length value to convert

How We Calculate This

This converter uses the exact definition: 1 inch equals exactly 25.4 millimetres.

The Formula

Inches to mm: mm = inches x 25.4
Mm to inches: inches = mm / 25.4

Common Automotive Conversions

15 inches (wheel) = 381 mm
17 inches (wheel) = 431.8 mm
19 inches (wheel) = 482.6 mm
1/4 inch = 6.35 mm
1/2 inch = 12.7 mm

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Actual figures may vary based on driving conditions, vehicle specifications, and other factors.