Inches to Millimeters Converter: Quick & Easy Conversion

The exact conversion

1 inch = 25.4 millimeters — not an approximation. Since 1959, the international yard and pound agreement has defined the inch as exactly 25.4 mm, so this multiplication carries no rounding error at all. Enter 1 and the tool returns 1 inches = 25.40 millimeters; enter 3.5 and you get 3.5 inches = 88.90 millimeters.

Why this differs from other length converters

Most imperial-to-metric factors on this site (cubic feet, acres, ounces) are rounded approximations of an irrational or long-decimal ratio. This one isn't — 25.4 is the legal definition, so machining and engineering drawings that spec dimensions in both units rely on this exact figure rather than a rounded stand-in.

Quick reference