1. Whenever there is a decimal point, all the zeros are significant.
E.g., 456.00 = 5 significant digits
2. When deciding the number of significant digits for multiplication and division, the number of the smallest number of significant digits in the question determines the smallest number of significant digits in the answer.
E.g., 6.0 x 11.224 = 67.344, the answer is = 67
3. Initial zeros are not significant (because they are just indicating the location of the decimal point).
E.g., 0.0063 meters has only two significant digits (63)
4. When adding or subtracting measurements, round up or down according to the measurement with the smallest number of decimal places.
E.g., 7.0 + 11.345 = 18.345, the answer is 11.3