Significant figures are the digits in a measurement or calculation that show the degree of precision. Here are the rules for determining how many significant figures are in a measurement:
1. All non-zero digits are significant.
2. Zeros that are in between non-zero digits are signifiicant.
3. Zeros at the end of a number are only significant if there's a decimal point.
4. Zeros at the beginning of a number aren't significant.
5. If a number is in scientific notation all of the digits in the coefficient are significant.
Here are some examples:
100 has one sig fig (rule 3)
1001 has four sig figs (rule 2)
0.00134 has three sig figs (rule 4)
23.5 has 3 sig figs (rule 1)
2.40 x 10^4 has 3 sig figs (rule 5)
When recording a measurement that you made you should include all of the certain digits plus one that is estimated. For example. if you're using a ruler marked off in millimeters, you estimate the last place so your measurement will be reported to the nearest .1 mm.