According to the Oxford Dictionaries, one millimeter is a unit of measure for length that is equal to 0.039 inches. Therefore, 25 millimeters is equal to 25 x 0.039, which is equal to 0.975 inches.
The more common conversion is the inch into centimeters, in which 1 inch is equal to 2.54 centimeters. In other words, one centimeter is equal to 1/2.54, which is equal to 0.39. But 1 centimeter is equal to 10 millimeters, which means that to find the number of millimeters that go into 1 inch, 0.39 should be divided by 10. The answer is 0.039, which is the same number provided by Oxford Dictionaries.