The total area of 12 millimeters by 10 millimeters is 120 square millimeters, assuming that this is an enclosed area with two sides of 12 millimeters and two sides of 10 millimeters. The formula for finding the area of a rectangle is length times width.
In this instance, 12 is multiplied by 10 and millimeters by millimeters. Whenever a unit of measurement is multiplied by itself, the answer is the unit squared. One inch is the equivalent of 25.4 millimeters, so the area described above is the equivalent of approximately 0.183 square inches. Dividing 12 millimeters by 25.4 millimeters per inch yields 0.472 inches, and dividing 10 millimeters by 25.4 millimeters per inch gives 0.394 inches. Multiplying the two values in inches together yields 0.183 square inches.