To convert inches to thousandths of an inch, multiply the inches by 1,000. By definition, "thousandths" is the division of an inch into 1,000; "MIL" is the standard abbreviation of this unit of measurement.
For example, multiply 5.25 inches by 1,000 to get 5,250 MIL, or multiply 10.5 inches by 1,000 to get 10,500 MIL. To convert MIL to inches, reverse the process by dividing a number of MIL by 1,000. For example, dividing 36,000 MIL by 1,000 results in 36 inches, which is also equal to 3 feet. Similarly, 456,000 MIL is equal to 456 inches or 38 feet.