How do you round a decimal to the nearest thousandth?

Answer 1

The thousandths place is three places to the right of the decimal point. If the digit right after that (in the ten-thousandths place) is 4 or lower, round down, i.e. erase everything but the first three digits after the point. If it's 6 or higher, round up, i.e. add 1 to the third digit. If it's 5, ask your teacher.

If that answer is unsatisfying, you can either (1) round up all 5's, or (2) round down if there are only 0's after it. This is the SI method, used for almost all scientific purposes.