The equivalent of 140 millimeters is 5.51 inches. That number can be determined either by directly converting from millimeters to inches, or by first converting the millimeters to centimeters, and then converting the centimeters to inches.
To directly convert between millimeters and inches, the number of millimeters is multiplied by 3.94. Because there are 10 millimeters in every centimeter, converting from millimeters to centimeters involves dividing the number of millimeters by 10. To then convert to inches, the centimeters are multiplied by 0.39 because there are 0.39 centimeters in every inch. Millimeters are the smallest unit of length in the metric system.