Significant Figures
The Rules
Significant figures are the way we tell if the number means something or it is just “holding a place” for the
numbers that do mean something. If it isn’t important, we say it is not significant. If it is important, we say it is
significant. We will talk more about why they are important tomorrow. Today, just learn how to use the rules.
This is how to tell if a number is significant
Rule 1: A number that isn’t a zero is ALWAYS significant
Example:
2389 has 4 significant figures.
12 has 2 significant figures.
899782 has 6 significant figures
Rule 2: A zero is sometimes significant. Here is how to tell if it is or not.

If it is “trapped” between 2 numbers, it is significant. This doesn’t matter if it is in the decimal or
a whole number.
Example:

If is in a decimal and it is after the numbers it is significant. This is like what we talked about
yesterday. You may know that it is 12.0 for sure, so you write 12.00. Those zeros stand for
how certain you are of your answer.
Example:

0.0009 has 1 significant figure
0.03090 has 4 significant figures
0200.001 has 6 significant figures
In whole numbers if the zero is after all the numbers and there is a decimal place, it is
significant.
Example:

12.00 has 4 significant figures
.90010 has 5 significant figures
.990 has 3 significant figures.
If it is in a decimal or a whole number and before any of the numbers it isn’t significant. These
zeros are just taking up space so the numbers that mean something are in the correct location.
Example:

101 has 3 significant figures
999.01 has 5 significant figures
.900009 has 6 significant figures
1000. has 4 significant figures
120. has 3 significant figures
90900. has 5 significant figures
In whole numbers if the zero comes after all the numbers and there is not a decimal place, it is
not significant
Example:
1000 has 1 significant figure
120 has 2 significant figures
90900 has 3 significant figures
Significant Figures
Practice
Step 1:
Determine how many significant figures are in each of the following numbers. Place an “X” over any
insignificant numbers. Write your answer for the total number of significant figures in the blanks.
Step 2:
Convert all numbers into scientific notation. If the zero is significant, it has to be written in the “number” part of
the scientific notation. (Example: 100. has 3 sig figs, so scientific notation is 3.00 x 102. I kept the significant
zeros. 100 has 1 significant digit, so scientific notation is 1 x 102. I dropped the insignificant zeros)
Number of Significant Digits
Scientific Notation
1. 3430.01
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2. 2020
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3. 0.00909220
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4. 2.90
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5. 52,000.
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6. 52,000
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7. 89930.090
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8. 0.0066270
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9. 0.5890
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10. 9,077,000
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11. 0.000088
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12. 9892
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13. 580.0
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14. 79010
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15. 9009
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16. 0.90040
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17. 10.000009
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18. 87.0220
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19. 83930.0
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20. 100,100,100
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