Physical measurements necessarily yield approximations. Although we can
measure some lengths with astonishing accuracy, we can't measure any
perfectly. Some inexactness, if only one part in thousands, remains. If
we try to get too fine with determining length, say of an iron rod, our
objectlevel notion of rigid boundaries fails. We reach a level of atomic
structure where the "iron rod" is in flux without exact boundaries.
 Precision
 Refers to the number of significant digits and/or
decimal places that can be reliably determined with a given
instrument or technique.
 Accuracy
 Refers to how close a measured value is to the "true"
or correct value.
 Significant figures
 Those figures in a measurement that are
known with reasonable accuracy.
The following rules apply to determine how many significant figures a measurement has:
 All digits other than zero are significant.
 Zeros between nonzero digits are significant.
 Leading zeros in a number are not significant.
 Trailing zeros in a number may or may not be significant.
Use standard form when appropriate to avoid confusion.
Here are some examples of significant figures:
327  three significant digits

2.09  three significant digits

0.000000382  three significant digits

16.83  four significant digits

3.2  two significant digits

3.20  three significant digits

150,000,000  not clear how many significant digits. Use scientific notation to clarify.

1.5 × 10^{8}  two significant digits

1.50 × 10^{8}  three significant digits

When rounding off measurements:
 When the digit immediately to the right of the last digit to be
rounded is less than five, the last digit is unchanged.
 When the digit immediately to the right of the last digit to be
retained is greater than five (or is equal to five, with nonzero
digits following), the last digit retained is increased by one.
 When there is only one digit to the right of the last digit to
be retained, and it is 5, the last digit to be retained remains
unchanged if even and is increased by one if odd.