Significant Figures

Significant Figures

Significant Figures are the number of digits that express the result to the true measured precision. For example, let's consider the number:

92.154

Determining the number of significant figures in this number is very simple. Start from the left and count the digits. In this case we find that there are 5 significant figures in this number.

Zeroes, however, are special digits. Sometimes they count as a significant digit, and sometimes they do not count and simply act as place holders. For example, the number

0.092067

has 5 significant figures. The first two zeroes are not significant, they are just place holders. Therefore, when you're counting significant digits always start from the left and don't start counting until you come to the first non-zero digit, then everything after that counts, including zeroes!

A good way to avoid the problem of significant and non-significant zeroes it is to use scientific notation. For example,

0.092067is written9.2067 x 10-2and has 5 sig. figs.
0.092 is written9.2 x 10-2 and has 2 sig. figs.
0.0920 is written9.20 x 10-2 and has 3 sig. figs.

When using scientific notation all digits, including zeroes, are significant.

  • Number of Significant Figures:

Rounding to the Correct Number of Significant Figures

In lab you were asked to weigh a crucible and cover three times. Let's say you measured

12.4337 grams, 12.4334 grams, and 12.4335 grams.

These numbers have six significant figures, but clearly the precision is poorer for the last digit. If I calculate the average I get

(12.4337 + 12.4334 + 12.4335) / 3 = 12.4335333333 ← calculator display

When I report this number it makes no sense to write down all the numbers on my calculator display because the average can have no more significant figures than what I get from a single measurement. We need to round off the number to the correct number of significant figures. In this case

12.4335333333 rounded to six sig. figs. is 12.4335

Generally, when insignificant digits are dropped from a number, the last digit retained should be rounded for the best accuracy. For example,

significant figure roundoff

To determine how the last digit should be rounded, the digits to be dropped are turned into a decimal fraction. In the example above, our decimal fraction would be 0.333333. We can now follow the rules for rounding numbers:

Rules for Rounding numbers to the correct number of significant figures.

  1. If the decimal fraction is greater than 1/2, then add one to the last digit retained.
  2. If the decimal fraction is less than 1/2, then leave the last digit retained alone.
  3. If the decimal fraction is exactly 1/2, then add one to the last digit retained only if it is odd.

Let's look at some examples.

Round the numbers 9.473, 9.437, 9.450, and 9.750 to two significant figures.

  • For 9.473 the last digit retained is 4, and the decimal fraction is 0.73. So we use rule #1 above and 9.473 is rounded to 9.5
  • For 9.437 the last digit retained is 4, and the decimal fraction is 0.37. So we use rule #2 above and 9.437 is rounded to 9.4
  • For 9.450 the last digit retained is 4, and the decimal fraction is 0.50. So we use rule #3 above and 9.450 is rounded to 9.4
  • For 9.750 the last digit retained is 7, and the decimal fraction is 0.50. So we use rule #3 above and 9.750 is rounded to 9.8
  • Rounding Numbers: