Convert Millimeters to Inches

About Millimeters to Inches Conversion

Millimeters (mm) and inches (in) are members of the International System of Units (SI) and Imperial / US customary systems, respectively. Although they are not very similar to each other in size, an inch containing approximately twenty-five millimeters, they are frequently used in similar applications. This is especially true of the fastener industry.

A selection of wrenches sold in a country where metric and imperial systems are both in common use will contain sizes in millimeters and fractions of an inch. At the scale of smaller fasteners in particular it is more convenient to use whole numbers of millimeters for sizing rather than decimals of centimeters. Centimeters being closer in size to the inch.

For unit conversion 1 inch contains exactly 25.4 millimeters. Given a quantity of millimeters dividing by 25.4 will yield the precise number of inches. To get millimeters from inches reverse the operation and multiply inches by 25.4.

The basis for this conversion is found in the international yard and pound agreement signed July 1, 1959 by six nations. The United States, the United Kingdom, Canada, Australia, New Zealand, and South Africa all participated in this agreement to set exact conversions for length and mass between the metric and imperial systems.

Units within the metric and imperial systems have defined ratios that establish relationships between them.

In the case of inches there are exactly 12 inches in a foot, there are 3 feet in a yard. By this relationship we know that there are 36 inches in a yard and because a yard is exactly 0.9144 meters according to the 1959 agreement a relationship between all units of length can be established.

Cautions

A millimeter is considerably smaller than an inch being approximately ¹/₃₂ of an inch (using common halving division of an inch). Therefore, it is most useful when measuring things that are fractions of an inch. In the metric system the centimeter is closer in size to the inch; 1 inch containing approximately 2.5 centimeters.

Millimeters and fractions of inches are often used to measure wrench sizes, nuts, bolts, and other hardware. Most countries measure fastening hardware in millimeters, the United States often measures hardware in fractions of an inch or mixed fractions for larger hardware.

It is natural to make hardware in sizes that correspond to whole numbers of millimeters and simple fractions of inches. Although each system has sizes that are closely aligned the natural breaks are too far apart to make the hardware interchangeable between metric and imperial / US customary sizes.

Common Metric and Imperial Wrench Sizes

Although hardware between the two systems is not compatible the table below compares some of the closest sizes for small fasteners.

Reference

• Canadian Journal of Physics, 1959, 37(1): 84, 10.1139/p59-014