 Measurement Fundamentals - 3: Inch Metric Conversions WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor. One inch is exactly 25.4 millimeters in length. But that was not always the case. Since both the inch and the meter were derived independently it is reasonable to wonder how the inch could be described so precisely by the meter. Prior to 1959 there was no exact conversion. An inch in relation to a meter was not defined past four decimal places. In that year and by international convention the inch gave up some of it's length so that it could be defined by the meter. Therefore the certainty of exactly 25.4 millimeters per inch today. If one inch is exactly 25.4 millimeters then what would the length of one millimeter be in inches? 1/25.4 = 0.03937007874015748 (and it goes on forever in decimal length) But, 0.0394 is close enough for most machine shop purposes. Some common conversions 1.0 millimeter = 0.0394 inch * 1.0 centimeter = 0.3937 inch * 1.0 decimeter = 3.937 inch * 1.0 meter = 39.37 inch * 1.0 inch = 25.4 millimeters ** 1.0 foot = 30.48 centimeters ** 1.0 yard = 0.9144 meters ** * approximate ** exact label: linear, angular, measurement, systems, inch, metric, convert, fractions, machinist inch