Seventeen millimeters is the equivalent of 0.669 inches, or slightly more than 2/3 inch. One inch equals 2.54 centimeters and, therefore, 25.4 millimeters. Dividing 17 millimeters by 25.4 millimeters per inch gives the result of 0.669 inches.
Conversions between the metric system and the system used in the United States are potentially somewhat involved. Converting within the metric system, however, often requires simply the moving of a decimal point to the right or left. For instance, 1 decimeter equals 10 centimeters or 100 millimeters. The decimal point just moves one place to the right each time as the unit of measurement goes from larger to smaller. When the unit of measurement goes from smaller to larger, the decimal point moves to the left.