When you need this conversion
Tools, fasteners, drill bits, jewelry and product specs. Much hardware worldwide is specified in mm, while US and some UK tools and stock are sized in fractions of an inch.
Nearest fractions
- 3 mm ≈ 1/8 in (3.175 mm)
- 6 mm ≈ 1/4 in (6.35 mm)
- 10 mm ≈ 3/8 in (9.525 mm)
- 13 mm ≈ 1/2 in (12.7 mm)
- 19 mm ≈ 3/4 in (19.05 mm)
- 25 mm ≈ 1 in (25.4 mm)
“Close” isn’t “equal”: the differences matter for bolts, bearings and tight fits.
Common mistakes
Swapping near-equivalent drill bits. A 1/4 in bit is 0.35 mm larger than a 6 mm bit, enough to make a screw hole too loose.
Confusing mm with cm. Divide cm by 2.54 but mm by 25.4.
For the reverse, use inches to mm. For larger lengths, use cm to inches.