Significant digits measure overall relative accuracy of a value.
In measurement, the last significant digit is the first one you have to estimate.
For example, if you were measuring something with a ruler marked in centimeters, and found that it was 15 cm long and just a little bit more, you would have to estimate the millimeters – say 15.2 cm. So the digit corresponding to millimeters (in this case the tenths place) would be the last significant digit.
How many significant digits in a number?
To count the number of significant digits in a number, do this:
1) Count all the non-zero digits.
2) Count any zero that has some non-zero digit to its left.
3) Don't count any other zeros.
So, the number 0.000405100 has six significant digits. (The first four zeros don't count; the other three zeros do, by rule 2.)