Length Conversion

Convert in to mm

1 in = 25.4 mm

25.4mm

Machined part tolerances and fastener sizes in US practice are dimensioned in inches, often down to thousandths; the same parts from metric-standard manufacturers are dimensioned in millimeters.

The inch is defined by the same 1959 international agreement as the pound: exactly 25.4 millimeters. Like the pound-to-kilogram conversion, this is a definition, not a derived approximation.

A 1/4 inch bolt (0.25 in) is 6.35 mm - close enough to a metric M6 bolt (6 mm) that the two are sometimes mistakenly swapped.

Reference Table

inches (in)	millimeters (mm)
1	25.4
5	127
10	254
25	635
50	1270
100	2540

Used In These Calculators

Frequently Asked Questions

Why is 25.4 mm per inch an exact number rather than a rounded approximation?

Because it was fixed by international agreement in 1959 (the "international inch"), specifically to end small discrepancies between slightly different inch definitions that had existed in different countries before then. Since that agreement, 1 inch = 25.4 mm exactly, by definition rather than by measurement.

How do you convert a fractional inch size, like 3/8 inch, to mm?

Convert the fraction to a decimal first, then multiply by 25.4 - 3/8 = 0.375 in, so 0.375 × 25.4 = 9.525 mm. This two-step approach (fraction to decimal, then to mm) is the standard way to handle the fractional inch sizes common on drill bits, fasteners, and lumber dimensions.

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