MM to Inches Converter

1. Type the length in millimeters. A caliper reading such as 12.7 or a size stamped on a bit such as 6 both work.
2. Pick the nearest fraction you can actually read: 1/16 in for a tape measure, 1/32 in for a steel rule, 1/64 in for a drill index.
3. Read the decimal inches, then the fraction and its error. A plus error means the fraction is larger than your part; a minus error means it is smaller.

Each result answers a different workshop question:

A millimeter to inch conversion expresses a metric length in inches by dividing by 25.4. The millimeter is one thousandth of a meter, the SI base unit of length. The inch is a US customary and UK imperial unit, equal to 1/36 of a yard.

The link between them is fixed by definition, not by measurement. Since 1959 the international yard is 0.9144 m, which makes the inch exactly 25.4 mm. NIST SP 811 lists the inch as 2.54 x 10^-2 m, and the UK Weights and Measures Act 1985 sets the yard at 0.9144 metre with the inch as 1/36 yard. The inch is therefore the same length in the United States and in the United Kingdom.

Most people who search this conversion hold a metric part and an inch tool, or the other way round. Typical cases are a metric drill bit and an inch drill index, a metric bolt and an inch socket set, or a metric drawing and an inch tape measure. That is why this page gives the nearest fraction of an inch, not just a decimal.

The converter divides millimeters by 25.4, then rounds the inch value to the nearest fraction you choose.

1. Divide by 25.4. Example: 13 mm / 25.4 = 0.51181 in.
2. Multiply by the denominator and round. At 1/64: 0.51181 x 64 = 32.76, which rounds to 33, so 33/64 in.
3. Find the error. 33/64 in = 13.097 mm, so the fraction is 0.097 mm larger than the part.

Dividing by 25.4 rarely gives a short decimal. 1 mm is 0.0393700787 in, and the digits go on. The fraction step is where rounding happens, so the tool always shows the error it created.

A 6 mm drill bit. 6 / 25.4 = 0.2362 in. At 1/64 the nearest bit is 15/64 in (5.953 mm), 0.047 mm smaller. At 1/16 it rounds to 1/4 in (6.35 mm), which is 0.35 mm larger. For a clearance hole that gap is fine; for a tight press fit it is not.

A 13 mm bolt head. 13 / 25.4 = 0.5118 in. A 1/2 in socket is 12.7 mm, 0.3 mm too small to fit. The 1/64 answer, 33/64 in, is 0.097 mm larger, but inch socket sets do not come in 64ths, so a 13 mm socket is the right tool.

A 19 mm wrench. 19 / 25.4 = 0.7480 in. The nearest 1/16 is 3/4 in (19.05 mm), only 0.05 mm larger. This is one of the closest metric and inch pairs.

A 250 mm shelf board. 250 / 25.4 = 9.8425 in. On a tape read to 1/16 that is 9 13/16 in, 0.763 mm short. Read to 1/32 it is 9 27/32 in, just 0.031 mm long. For measuring tape work in longer lengths, the inches to feet converter splits the result further.

A 0.8 mm sheet. 0.8 / 25.4 = 0.0315 in, or 31.5 thou. The nearest 1/64 is 1/32 in (0.794 mm), 0.006 mm thin.

The decimal inch value never changes. The fraction you read does change with three things.

The Precision You Choose

A 1/16 in step is 1.5875 mm, so the worst rounding error is half of that: 0.794 mm. A 1/32 in step halves the worst case to 0.397 mm. A 1/64 in step cuts it to 0.198 mm. Pick the finest step your tool actually marks.

Where the Length Falls Between Marks

Some lengths land almost on a mark. 8 mm is 0.062 mm over 5/16 in, and 19 mm is 0.05 mm under 3/4 in. Others land halfway: 7 mm is 0.65 mm over 1/4 in at 1/16 precision. The error column shows which case you have.

How the Tool Set Is Sized

Drill indexes often step in 1/64 in, while many socket and wrench sets step in 1/16 in. A 1/64 answer is useless for a set that only has sixteenths, so match the precision to the set.

Measurement Uncertainty

A caliper reading of 10.0 mm may really be 9.98 mm. The conversion is exact, but it cannot be more accurate than the reading you type in.

Finer fractions are more accurate but harder to read. This table compares the three settings.

NIST Handbook 44 lists the mil, or thou, as exactly 0.0254 mm. For work finer than 1/64 in, read the thousandths result instead of a fraction. For rounding the decimal to a set number of places, the rounding calculator shows each rounding rule.

Matching a Metric Drill Bit to an Inch Index

A metric plan calls for a 5 mm pilot hole, but the bit case is fractional. 5 mm is 0.1969 in, so 13/64 in (5.159 mm) is the nearest bit, 0.159 mm larger.

Choosing a Wrench or Socket

Check the error before you swap sizes. The smaller the gap, the closer the fit; a tool that is too large can slip and round off a bolt head. 10 mm is 0.475 mm bigger than 3/8 in, so a 3/8 in wrench will not fit a 10 mm nut.

Reading Metric Drawings with an Inch Tape

Furniture and cabinet plans from Europe are drawn in mm. Convert each cut length, then mark it at 1/16 or 1/32 in. For the full imperial picture across many units, use the imperial to metric converter.

Decimal inches and the nearest 1/16 and 1/64 inch, with the error in mm. A plus error means the fraction is longer than the metric length. Values are computed from 1 in = 25.4 mm.

1. Dividing by 25 Instead of 25.4

Dividing by 25 makes every answer 1.6 percent too large. At 500 mm that is 20 in instead of 19.685 in, an error of 8 mm.

2. Rounding to a Fraction the Tool Set Does Not Have

33/64 in is the best answer for 13 mm, but no standard socket set has a 33/64 in socket. Pick the precision that matches the set.

3. Ignoring the Sign of the Error

A fraction that is too small will not fit over a bolt head. A drill that is too small leaves a tight hole. The plus or minus sign tells you which way the fraction misses.

4. Rounding the Decimal Before Making the Fraction

Rounding 0.2362 in to 0.24 in first and then picking a fraction can move you to the wrong mark. The tool builds the fraction from the full value.

5. Treating Near Sizes as Equal

6 mm and 1/4 in differ by 0.35 mm. For a loose fit that is fine, but not for a bearing, dowel or tight bore.

The decimal result is exact to the digits shown, because the inch is defined as exactly 25.4 mm. The fraction is always rounded.

One millimeter is 0.0393700787 inches. Rounded, that is 0.0394 in, or about 39.37 thousandths of an inch.

Ten millimeters is 0.3937 in. The nearest 1/64 is 25/64 in, 0.078 mm short. To the nearest 1/16 it is 3/8 in, which is 0.475 mm short.

Almost. 19 mm is 0.748 in, and 3/4 in is 19.05 mm, so the gap is only 0.05 mm. It is one of the closest metric and inch wrench pairs.

Six millimeters is 0.2362 in. The nearest 1/64 is 15/64 in, which is 0.047 mm smaller. To the nearest 1/16 it rounds to 1/4 in, 0.35 mm larger.

Pick the finest mark your tool shows. Use 1/16 in for a tape measure, 1/32 in for a steel rule and 1/64 in for a fractional drill index.

Divide by 25, or multiply by 4 and move the decimal two places left. That gives 0.4 in for 10 mm, 1.6 percent high against the true 0.3937 in.

One millimeter is 39.37 thou. A thou, also called a mil, is 0.001 in, which NIST defines as exactly 0.0254 mm.

The inch is 25.4 mm, so 25 mm is 0.9843 in, about 0.4 mm short. The nearest 1/64 is 63/64 in.

Divide by 25.4 for inches, then divide by 12. For 1,000 mm that gives 39.37 in, which is 3 ft 3.37 in.

Yes. Both use the international inch of exactly 25.4 mm, agreed in 1959. UK law defines the inch as 1/36 of a yard of 0.9144 metre.

• Conversion factors for the inch and the mil (NIST SP 811, Appendix B.8).
• Tables of US customary length units and thickness in mils (NIST Handbook 44, 2026, Appendix C).
• The yard of 0.9144 metre and the inch as 1/36 yard (Weights and Measures Act 1985, Schedule 1, legislation.gov.uk).
• The 1959 international foot and inch definitions (NIST Office of Weights and Measures).