Many jewelry dimensions are described in millimeters. A millimeter is approximately equal to 0.04 inches.
To convert inches to millimeters:
mm = inches/.04
Example to convert 1 inch to millimeters:
1 inch/.04 = 25 millimeters.
To convert millimeters to inches:
inches = millimeters x .04
Example to convert 25 millimeters to inches:
25 millimeters x .04 = 1 inch.
For reference, here are some millimeter dimensions of objects with which you are familiar.