1 millimeter equals
0.039370079 inches.
Enter a value below to convert instantly.
Millimetres are the working unit of engineering drawings, hardware specs and machine shops outside the US, and they map onto inches through the same exact 25.4 factor that governs every other metric-to-Imperial length. This is the conversion you reach for when a metric drawing has to be cut, drilled or bought against an inch-based tool set.
1mm=
0.039370079in
Multiply the number of millimeters by 0.0393700787402 to get inches.How this is calculated
Copy link puts your number after the #, which browsers never send to a server — but the link itself carries it, so anyone you send it to sees the value. Save keeps the unit pair in this browser only; clear it any time from your saved list.
Multiply the number of millimeters by 0.0393700787402 to get inches.
Step by step
Start with 1 mm.
Multiply by the conversion factor 0.0393700787402.
1 × 0.0393700787402 = 0.039370079 in.
How to convert millimeters to fractional inches
Rulers, tape measures and drill bits are marked in halves, quarters,
eighths and sixteenths, so a decimal answer usually has to be snapped
to the nearest mark before you can use it. Convert to decimal inches
first, then multiply the part after the decimal point by 16 and round
to a whole number — that is your number of sixteenths. Reduce the
fraction if it divides evenly.
One millimeter is 0.039370079 in, which is
1/16 in to the nearest
sixteenth, or 3/64 in to the
nearest sixty-fourth. The fraction column in the table below is
rounded to the nearest sixteenth.
Millimeters to Inches conversion table
Conversion table from Millimeters to Inches
Millimeters (mm)
Inches (decimal)
Inches (fraction)
1
0.039370079
1/16
2
0.078740157
1/16
3
0.11811024
1/8
5
0.19685039
3/16
10
0.39370079
3/8
20
0.78740157
13/16
50
1.9685039
1 15/16
100
3.9370079
3 15/16
500
19.685039
19 11/16
1,000
39.370079
39 3/8
Where millimeters and inches are used together
Fasteners and hardware
Metric bolts are specified by shank diameter in millimetres (M6, M8, M10) while US hardware is fractional-inch (1/4, 5/16, 3/8). They are close enough to start threading and different enough to strip — an M8 is 8 mm, a 5/16 inch bolt is 7.94 mm, and neither belongs in the other's hole.
Machining and 3D printing
CAD files circulate in both systems. A part drawn in inches and sliced in millimetres will print at the right size only if the conversion happened at import; a silent unit mismatch shows up as a part exactly 25.4 times too big or too small.
Drill bits and tapping
Tap drill charts are published in both systems, and the nearest metric bit to a fractional one is often within a few hundredths of a millimetre. Converting first tells you whether 'close enough' really is — for a clearance hole it usually is, for a tapped hole it usually is not.
Rainfall, tyres and thickness specs
Tyre widths, sheet metal gauges and rainfall totals are all quoted in millimetres internationally. A 225 mm tyre and an 8.86-inch tyre are the same tyre, but only one of those numbers appears on the sidewall.
Everyday millimeters worth knowing
Values people actually look up, with the reason each one comes up. Every
figure is calculated with the same engine as the converter above.
Millimeter (mm)
Inch (in)
What it is
1
0.039370079
The base conversion
6
0.23622047
A common metric bolt diameter (M6)
10
0.39370079
One centimetre
12.7
0.5
Exactly half an inch
25.4
1
Exactly one inch, by definition
100
3.9370079
A hand's width
210
8.2677165
The short side of A4 paper
297
11.692913
The long side of A4 paper
How these two units are defined
A millimetre is one thousandth of a metre. An inch is exactly 25.4 millimetres, which follows directly from the exact 2.54 cm definition. In practice millimetres and inches are used at different scales: millimetres for anything you would machine or fasten, inches for anything you would buy off a US shelf. The unit that carries the tighter tolerance in a drawing is usually the one the part was designed in.
Rounding and precision
One thousandth of an inch is 0.0254 mm, so a millimetre value quoted to two decimal places is finer than a typical shop tolerance and three decimals is almost always false precision. The reverse trap is worse: converting a whole-millimetre dimension into inches produces long decimals (7 mm becomes 0.2755905… in), and copying all of those digits into a drawing implies a tolerance nobody can hold. Round to the precision the part actually needs, and state the tolerance separately.
Mistakes people make with this conversion
Treating M6 and 1/4 inch as interchangeable
A quarter inch is 6.35 mm against M6's 6 mm. The bolt will enter and it will not hold thread properly. The same trap catches M8 against 5/16 inch and M10 against 3/8 inch.
Importing CAD without setting units
A model that arrives 25.4 times too large or too small is almost never corrupt — it is an inch file read as millimetres or the reverse. Check one known dimension before you scale anything.
Rounding 25.4 to 25
That is a 1.6% error, which over a 300 mm run is nearly 5 mm. Fine for a rough sketch, disastrous for a hole pattern.
Exactly 25.4. The value is a definition rather than a measurement, so it carries no uncertainty and needs no extra decimal places.
What is 10 mm in fractional inches?
10 mm is 0.3937 in, which is between 6/16 (0.375) and 7/16 (0.4375) of an inch — closest to 25/64. This is why a 10 mm socket almost fits a 3/8 inch fastener but rounds off the corners if you force it.
Which is more precise, a millimetre or an inch?
Neither unit is inherently more precise; precision comes from the instrument. But because a millimetre is about 1/25 of an inch, a scale marked in whole millimetres is roughly as fine as one marked in 32nds of an inch.
How many inches are in a millimeter?
One millimeter equals 0.039370079 inches.
What is 1 millimeter in inches as a fraction?
One millimeter is 1/16 inch to the nearest sixteenth, or 3/64 inch to the nearest sixty-fourth — the marks you will find on a tape measure.
How do you convert millimeters to inches?
Multiply the number of millimeters by 0.0393700787402 to get inches.
What is 1 inch in millimeters?
One inch equals 25.4 millimeters.
When would I convert millimeters to inches?
This conversion is common when measuring height, screen sizes, travel distances, fabric, and construction dimensions.