What's 1/4 inch in mm?

0.25 inch equals 6.35 mm — close to but not the same as the metric 6 mm drill bit, which matters for tight tolerances.

Is the conversion factor exact?

Yes. By international agreement one inch is exactly 25.4 mm, so inch-to-mm conversions have no truncation error.

Convert inches into millimeter precision for drilling charts, fastener sizing, machining tolerances or specifying parts in metric.

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From

1 in = 25.4 mm

millimeters = inches × 25.4

What's 1/4 inch in mm?: 0.25 inch equals 6.35 mm — close to but not the same as the metric 6 mm drill bit, which matters for tight tolerances.
Is the conversion factor exact?: Yes. By international agreement one inch is exactly 25.4 mm, so inch-to-mm conversions have no truncation error.