Metric & Imperial Converter

Exact conversion, no rounding shortcuts

1 inch is defined as exactly 25.4 mm — this isn't an approximation, it's the legal definition both the US and metric world use, so there's no rounding error to worry about at any precision. "Thou" (thousandths of an inch, also called mils) is just the inch value multiplied by 1,000, useful for talking about small tolerances without a string of leading zeros.

Where mixed units cause real problems

A print dimensioned in mm read against a US-made gauge block or caliper set to inches (or the reverse) is one of the more common sources of an otherwise-avoidable scrapped part — always confirm which unit a print is actually calling out before cutting metal, especially on a mixed shop floor where jobs alternate between inch and metric customers.