Significant Figures

Significant figures (digits) are those integers (0-9) that can be assumed to be accurate for the measurement being made.
All nonzero numbers are considered significant.

The zeros are significant in only some cases.

For instance, the zeros in 1005 are considered significant because they define the size of the number and are surrounded by nonzero digits.

For the number 0.4020 the zero to the left of the decimal point is not significant, but the other two zeros are because they define the magnitude of the number and the fourth-place accuracy of the reading.