Rules For Determining If a Number Is Significant or Not
All non-zero digits are considered significant. For example, 91 has two significant figures (9 and 1), while 123.45 has five significant figures (1, 2, 3, 4, and 5).
Zeros appearing between two non-zero digits (trapped zeros) are significant. Example: 101.12 has five significant figures: 1, 0, 1, 1, and 2.
Leading zeros (zeros before non-zero numbers) are not significant. For example, 0.00052 has two significant figures: 5 and 2.
Trailing zeros (zeros after non-zero numbers) in a number without a decimal are generally not significant (see below for more details). For example, 400 has only one significant figure (4). The trailing zeros do not count as significant.
Trailing zeros in a number containing a decimal point are significant. For example, 12.2300 has six significant figures: 1, 2, 2, 3, 0, and 0. The number 0.000122300 still has only six significant figures (the zeros before the 1 are not significant). In addition, 120.00 has five significant figures since it has three trailing zeros. This convention clarifies the precision of such numbers. For example, if a measurement that is precise to four decimal places (0.0001) is given as 12.23, then the measurement might be understood as having only two decimal places of precision available. Stating the result as 12.2300 makes it clear that the measurement is precise to four decimal places (in this case, six significant figures).
The number 0 has one significant figure. Therefore, any zeros after the decimal point are also significant. Example: 0.00 has three significant figures.
Any numbers in scientific notation are considered significant. For example, 4.300 x 10-4 has 4 significant figures.
Conventions Addressing Significant Figures
The significance of trailing zeros in a number not containing a decimal point can be ambiguous. For example, it may not always be clear if a number like 1300 is precise to the nearest unit (and just happens coincidentally to be an exact multiple of a hundred) or if it is only shown to the nearest hundred due to rounding or uncertainty. Various conventions exist to address this issue:
A bar may be placed over the last significant figure, showing that any trailing zeros following this are insignificant. For example, 1300 with a bar placed over the first 0 would have three significant figures (with the bar indicating that the number is precise to the nearest ten).
The last significant figure of a number may be underlined; for example, "2000" has two significant figures.
A decimal point may be placed after the number. For example "100." indicates specifically that three significant figures are meant.
In the combination of a number and a unit of measurement the ambiguity can be avoided by choosing a suitable unit prefix. For example, the number of significant figures in a mass specified as 1300 g is ambiguous, while in a mass of 13 hg or 1.3 kg, it is much clearer.
When converting from decimal form to scientific notation, always maintain the same number of significant figures. For example, 0.00012 has two significant figures, therefore the correct scientific notation for this number would be 1.2 x 10-4.
When multiplying and dividing numbers, the number of significant figures used is determined by the original number with the smallest amount of significant figures. When adding and subtracting, the final number should be rounded to the decimal point of the least precise number.
1.423 x 4.2 = 6.0 since 1.423 has 4 significant figures and 4.2 only has two significant figures, the final answer must also have 2 significant figures.
234.67 - 43.5 = 191.2 since 43.5 has one decimal place and 234.67 has two decimal places, the final answer must have just one decimal place.
Another Way to Determine Sig Figs: The Pacific Rule & the Atlantic Rule
It can be challenging to remember all the rules about significant figures and whether each zero is significant or not significant. Here's another way to determine significant figures (sig figs): the Pacific and Atlantic Rule.
If a number has a decimal Present, use the Pacific rule (note the double P's). The Pacific Ocean is on the left side of the United States so start at the left side of the number. Start counting sig figs at the first non-zero number and continue to the end of the number. For example, since there is a decimal present in 0.000560 start from the left side of the number. Don't start counting sig figs until the first non-zero number (5), then count all the way to the end of the number. Therefore, there are 3 sig figs in this number (5,6,0).
If a number has no decimal (the decimal is Absent) use the Atlantic rule (again, note the double A's). Since the Atlantic Ocean is on the right side of the United States, start on the right side of the number and start counting sig figs at the first non-zero number. For example, since there is no decimal in 2900 start from the right side of the number and start counting sig figs at the first non zero number (9). So there are two sig figs in this number (2,9).