How Do You Convert Millimeters (mm) to Inches?

Millimeters belong to the metric system and inches belong to the imperial system. One inch equals 25.4 millimeters, so you convert by dividing the millimeter value by 25.4. This page gives you the formula, a worked example, a full conversion chart, the fraction form used on tools, and the reason the 25.4 figure is exact.

What Are Millimeters and Inches as Units of Length?

Both units measure length, but they come from two different systems. The millimeter is part of the metric system, which most of the world uses. The inch is part of the imperial and US customary systems, used mainly in the United States and still seen in the United Kingdom and Canada. Every conversion between them rests on one fixed link. One inch equals 25.4 millimeters. Once you know that single number, every result follows from it.

What Is a Millimeter (mm)?

A millimeter is one thousandth of a meter. It is written as mm. Ten millimeters make one centimeter, and one thousand millimeters make one meter. Because it is a small unit, it is used for fine and precise sizes, such as the thickness of a sheet, the width of a small part, or the gap in a machined fit. It is the metric unit that engineers and manufacturers reach for when accuracy matters.

What Is an Inch (in)?

An inch is a unit in the imperial and US customary systems. It is written as in, or with a double prime mark after the number. One inch equals 25.4 millimeters and 2.54 centimeters. There are twelve inches in a foot. Inches are often split into fractions rather than decimals, using halves, quarters, eighths, sixteenths, thirty-seconds, and sixty-fourths. The word inch comes from the Latin uncia, meaning one twelfth.

How Many Millimeters Are in One Inch?

One inch equals 25.4 millimeters exactly. This is not a rounded value. Turned around, one millimeter equals about 0.03937 of an inch. A common mistake is to treat 25 millimeters as one inch. They are close but not equal. 25 mm is about 0.984 inch, while a full inch needs 25.4 mm. That extra 0.4 mm matters in any job where a tight fit counts.

What Is the Formula to Convert Millimeters to Inches?

To convert millimeters to inches, divide the millimeter value by 25.4.

inches = millimeters / 25.4

You can also multiply by 0.0393701, which gives the same answer, since one millimeter is 0.0393701 inch. To go the other way, multiply the inch value by 25.4.

millimeters = inches x 25.4

Dividing by 25.4 works because there are exactly 25.4 millimeters inside every inch, so each group of 25.4 mm becomes one inch.

How Do You Convert Millimeters to Inches Step by Step?

Take 100 mm as an example.

1.     Start with the value in millimeters: 100.

2.     Divide it by 25.4: 100 / 25.4.

3.     Read the result: about 3.937 inches.

Round the answer to the precision you need. For a second example, 50 mm divided by 25.4 is about 1.969 inches. The result is rounded, so it is shown as an approximate value rather than an exact one.

How Do You Convert Millimeters to a Fraction of an Inch?

Tape measures and hand tools are marked in fractions, not decimals, so you often need the nearest fraction instead of a long decimal. The method has two steps. First, convert the millimeters to a decimal inch using the formula above. Then round that decimal to the nearest fraction, such as 1/16, 1/32, or 1/64.

For example, 10 mm equals about 0.3937 inch. On a sixteenths tape the closest mark is 3/8 inch, which is 0.375 inch, and on a finer scale 25/64 inch (0.3906 inch) is closer still. The fraction is always the nearest match, so it can be a little larger or smaller than the exact value, which leaves a small remainder. Finer fractions such as 1/64 give a closer match when you need more accuracy.

What Is the Millimeters to Inches Conversion Chart?

Use the chart below to read values quickly instead of doing the math each time. It lists the millimeter value, the decimal inch result, and the nearest fraction rounded to 1/64. The fractions are close matches rather than exact figures, except where marked exact.

What Are the Most Common Millimeter to Inch Conversions?

These are the single values people look up most often. Two of them are worth remembering on their own. 25.4 mm is exactly one inch, and 12.7 mm is exactly half an inch.

Why Is One Inch Defined as Exactly 25.4 Millimeters?

Before 1959, the inch had slightly different values in different countries. In 1959, the United States, the United Kingdom, Canada, and others signed the international yard and pound agreement, which set the inch at exactly 25.4 millimeters. This made the inch the same everywhere it is used and tied it directly to the metric system. That is why 25.4 is treated as an exact number today, and why every reliable converter and engineering standard uses it.

Where Do You Need to Convert Millimeters to Inches?

You meet this conversion any time metric and imperial sizes sit side by side.

•        In engineering and machining, plate thickness and tolerances are often listed in millimeters but checked against inch based tools.

•        In construction and piping, beams, panels, and pipe sizes may be given in either unit.

•        For drill bits and fasteners, a metric size often has to be matched to the nearest inch sized bit or wrench.

•        In jewelry, ring and stone sizes move between mm and inches.

•        For screens, television and display sizes are quoted in inches while the panel is built in millimeters.

•        For height and woodworking, plans switch between the two units.

Mixed drawings that show both units on one sheet are common, which is exactly why a quick converter saves time and prevents errors.

How Does Millimeter to Inch Conversion Fit Into the Metric and Imperial Systems?

Converting millimeters to inches is one link in a wider set of length conversions. The metric system scales in steps of ten, so millimeters, centimeters, and meters connect through simple multiples. The imperial system uses inches, feet, and yards, which connect through twelves and threes. Knowing how mm and inches relate makes the rest of these conversions easier to follow, since they all share the same idea of moving a value from one unit to another. The pages below cover the conversions most closely linked to this one.

How Do You Convert Centimeters to Inches?

To convert centimeters to inches, divide the centimeter value by 2.54, since one inch equals 2.54 cm. It is the same metric to imperial step as mm to inches, just one unit larger. For the full tool and chart, see the cm to inches calculator.

How Do You Convert Millimeters to Centimeters?

Within the metric system, millimeters and centimeters are simple to move between. Divide the millimeter value by ten to get centimeters, because there are ten millimeters in each centimeter. For more metric length steps, see the metric length converter.

How Do You Convert Feet to Inches?

To convert feet to inches, multiply the number of feet by twelve, since there are twelve inches in a foot. This pairs with mm to inches when a drawing lists feet, inches, and millimeters together. See the feet to inches calculator for the full tool.

How Do You Convert Meters to Feet?

One meter is about 3.281 feet. This is the larger metric to imperial length step that sits next to mm to inches. For the full conversion, see the meter to feet calculator.

How Do You Convert a Fraction to a Decimal?

To turn a fraction into a decimal, divide the top number by the bottom number. This is the same skill used to read a fractional inch as a decimal. For a quick tool, see the fraction to decimal calculator.

How Do You Read Decimal Inches on a Tape Measure?

A tape measure is marked in fractions, so a decimal inch result has to be matched to the nearest mark, usually 1/16 or 1/32. Knowing how the marks line up makes the fraction step above easier. See the decimal to inches and tape measure guide for the details.

What Is a Length Conversion Calculator?

A length conversion calculator handles any length unit, including millimeters, centimeters, meters, inches, feet, and yards, in one place. It is the parent tool for every length conversion on this site. See the length conversion calculator.

How Do You Convert Your Height Into Feet and Inches?

Height is often recorded in centimeters or millimeters but needed in feet and inches. To get there you convert to inches first, then split the inches into feet and remaining inches. See the height conversion calculator for a direct tool.