Millimetres to inches
Convert millimetres to inches — type any value and get the answer instantly.
The conversion
One millimetre is 0.0393700787 inches, and one inch is exactly 25.4 millimetres. As with centimetres this is a definition, fixed in 1959, not an approximation.
A quick rule: divide by 25 and you are within two percent.
Why millimetres dominate engineering
Almost all technical drawing outside the United States is dimensioned in millimetres, with no decimal point — a part is "12" rather than "1.2 cm". Whole numbers remove the ambiguity that costs money when a drawing is misread, which is exactly why the convention exists.
Fractional inches
The common workshop sizes convert to memorable millimetre figures: 1/16 in is 1.59 mm, 1/8 in is 3.18 mm, 1/4 in is 6.35 mm, and 1/2 in is 12.7 mm. Drill bits, timber and fasteners are all sold in one system or the other, and a metric bit is never exactly an imperial one — 6 mm is not 1/4 inch, it is 0.236 in, and forcing one where the other belongs is how threads get stripped.
Where it comes up
Phone screens, camera lenses, printing bleed, watch cases and 3D-printing tolerances are all quoted in millimetres, while much American hardware remains fractional-imperial.
Rounding, and when it matters
For most work, two decimal places is plenty. For machining it is not — a tenth of a millimetre is four thousandths of an inch, which is the difference between a part that fits and one that does not. When a tolerance is specified, convert the tolerance too rather than converting the nominal size and assuming the slack came with it.
Other units
The box takes anything — try 70 kg to lbs, 2 gb in mb or 60 mph to kmh.
Common questions
How do I type a conversion?
Plainly: 70 kg to lbs, 5 ft in cm, 180 c to f. Abbreviations and full names both work.
Is a kilobyte 1000 or 1024 bytes?
1,024 here, matching operating systems. Drive makers use 1,000, which is why a 1 TB disk shows as 931 GB.
Why is temperature different?
The scales do not share a zero, so it is a multiplication and an offset rather than a simple factor.
Can it convert kg to metres?
No — those measure different things, so it explains rather than returning a meaningless number.
Is it accurate?
Conversions use exact defined factors where they exist: an inch is exactly 2.54 cm, a pound exactly 0.45359237 kg.