The first significant figure (sf or
sig fig) is the first digit that is not
zero. The second, third, forth...
significant figures follow on after
the first digit. They may or may
not be zeros.
Examples:
2.7305 has 5 sf
0.00239 has 3 sf
To round a number to a given
number of significant places,
apply the same rules with
decimal places. If the next
digit is 5 or more, round up.