# Significant Digits

**
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
$\text{0}\text{.000405100}$
has
**
six
**
significant digits. (The first four zeros don't count; the other three zeros do, by rule
$2$
.)