MM to Inches Converter

Enter a measurement in millimeters to get inches as an exact decimal and as the nearest 16th-inch fraction, with the rounding difference shown honestly.

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How to convert mm to inches

A millimeter is a metric unit of length; an inch is a US and imperial unit. Since 1959 the two have been linked by one exact number, so converting between them is a single division (or multiplication in reverse), never a rounded approximation of the factor itself.

The formula

inches = millimeters ÷ 25.4   or   millimeters = inches × 25.4

25.4 is exact, defined, not measured. Any rounding you see in a result comes from how many digits you choose to keep, never from the factor.

Worked example

20 mm to inches:

20 ÷ 25.4 = 0.787402 in (6 significant figures). Rounded to the nearest 16th of an inch, that is 13/16 in (0.8125 in), which is 0.0251 in larger than the exact decimal, about 0.6375 mm oversize.

Reverse direction: 3/16 in (0.1875 in) to millimeters: 0.1875 × 25.4 = 4.7625 mm, exactly, with no rounding at all because the multiplication resolves to a finite decimal.

Reference table: mm to inches

mmInches (decimal)Nearest 16th
1 mm0.0393701 in1/16 in
2 mm0.0787402 in1/16 in
3 mm0.11811 in1/8 in
4 mm0.15748 in3/16 in
5 mm0.19685 in3/16 in
6 mm0.23622 in1/4 in
7 mm0.275591 in1/4 in
8 mm0.314961 in5/16 in
9 mm0.354331 in3/8 in
10 mm0.393701 in3/8 in
11 mm0.433071 in7/16 in
12 mm0.472441 in1/2 in
13 mm0.511811 in1/2 in
14 mm0.551181 in9/16 in
15 mm0.590551 in9/16 in
16 mm0.629921 in5/8 in
17 mm0.669291 in11/16 in
18 mm0.708661 in11/16 in
19 mm0.748031 in3/4 in
20 mm0.787402 in13/16 in
21 mm0.826772 in13/16 in
22 mm0.866142 in7/8 in
23 mm0.905512 in7/8 in
24 mm0.944882 in15/16 in
25 mm0.984252 in1 in
26 mm1.02362 in1 in
27 mm1.06299 in1 1/16 in
28 mm1.10236 in1 1/8 in
29 mm1.14173 in1 1/8 in
30 mm1.1811 in1 3/16 in

The exact factor: why 25.4 mm is exactly 1 inch

Before July 1, 1959, the US inch and the UK (imperial) inch were not quite the same length. Each country had defined its own inch slightly differently, and the two differed by about two parts per million, a gap of a few millionths of an inch that mattered in precision manufacturing, ballistics, and international trade. The International Yard and Pound Agreement, signed by the United States, the United Kingdom, Australia, Canada, New Zealand, and South Africa, fixed the international yard at exactly 0.9144 meters, which makes the inch exactly 25.4 mm everywhere those countries use it. It is one of the few unit conversions in daily use with zero uncertainty in the factor itself.

When precision matters: woodworking vs machining

A carpenter reading a tape measure works in 16ths of an inch because that is what the markings show, and a gap of a few hundredths of an inch is invisible in a stud wall or a shelf. A machinist cutting metal to a tolerance of 0.01 mm cannot afford that rounding: 1/16 in can be off from a true metric target by half a millimeter or more, enough to jam a bearing or misalign a bolt pattern. If your project is measured in metric drawings and needs to fit metric hardware, convert and work in millimeters throughout rather than round-tripping through 16ths. If you are marking a board with a US tape measure, the fraction is the more useful number, and this calculator always tells you exactly how far off that fraction is.

Related conversions

Working in centimeters instead? Try the CM to Inches Converter. Going the other way from inches? See Inches to CM Converter. Need feet instead of a fraction? Use the Inches to Feet Converter. Converting a person's height specifically, see the Height Converter, or for meters and feet see the Meters to Feet Converter.

Frequently asked questions

How many inches is 1 mm?

1 mm equals 0.0393701 in, to 6 significant figures. The exact fraction is 1/25.4 in, which does not land on a clean 16th-inch mark, so a ruler reading of 1 mm is closest to 1/16 in, oversize by about 0.023 mm.

How do I convert mm to inches?

Divide the millimeter value by 25.4, since 1 inch is defined as exactly 25.4 mm. For example, 50 mm / 25.4 = 1.9685 in. There is no rounding in the factor itself, only in how many digits you choose to keep.

Why is 25.4 mm exactly 1 inch?

Since 1959, the United States, the United Kingdom, and four other countries have defined the inch as exactly 25.4 mm under the International Yard and Pound Agreement. Before that date, the US and UK inches differed from each other by a few millionths of an inch, which mattered for precision manufacturing and international trade.

Is a mm-to-inches conversion ever exact as a fraction?

Rarely. Because 25.4 does not divide evenly into most millimeter values, the decimal inch result is almost always an infinite or long repeating decimal. Rounding to the nearest 16th of an inch is a practical approximation, not an exact equivalence, and this calculator always shows you the size of that gap.

Should I round to 16ths or use the decimal for machining?

For machining, 3D printing, or anything with a tolerance tighter than about 0.5 mm, use the decimal inch value or convert in millimeters directly. Sixteenth-inch fractions are a carpentry and tape-measure convention; the rounding error can be 0.4 mm or more, which is too coarse for metal parts or tight-fitting joints.

What is 3/16 inch in mm?

3/16 in equals 4.7625 mm exactly, since 3/16 times 25.4 is an exact multiplication with no rounding involved. Switch this calculator to the inches to mm direction and enter 3/16 directly; fractions like 1/8, 3/16, and mixed numbers like 1 1/4 are all accepted.

How many mm are in an inch?

There are exactly 25.4 mm in 1 inch, by international definition since 1959. This is an exact conversion factor, not a rounded approximation, so any precision loss in a result comes only from how you round the final answer.

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