Significant Figures
Not all numbers matter…
Significant Figures
• Definition: Digits of a number that are
relevant when doing mathematical
calculation.
• Significant figures are useful so we know when
to “cut off” the amount of digits used in
answering a problem.
• Just because it’s on the calculator, doesn’t
mean it has to be part of the written answer…
Rules for Significant Figures
• Most figures are significant
• There are 2 basic rules for when a digit, or
“figure” in a number isn’t significant.
• Rule #1: If a “0” is:
a) at the end of a number AND
b) THERE’S NO DECIMAL POINT
then those 0’s aren’t significant.
Rule #1- Examples
Some of the digits here are significant and some
are not:
significant NOT significant
240,000
Significant
5024
Significant
200.300
Rules for Significant Figures
• Rule #2: If a “0” is at the BEGINNING of a
number, it is NOT significant
Rule #2- Examples
Some of the digits here are significant and some
are not:
NOT significant
significant
0245.08
NOT Significant significant
0.000005024
Practice
• On your white board, write down the number
of significant digits for each number, then hold
your board up facing the teacher.
Practice
23061
Practice
26,000
Practice
26,000.0
Practice
26,000.030
Practice
03.14
Practice
0.00314
Using Significant Figures
• We use significant figures in math to know
how “much answer” to use.
• For Example: 2  0.666
3
• The number goes on forever, you need to
know how many decimal places to use.
Addition and Subtraction
• The answer has the least amount of numbers
after the decimal as the numbers you are
adding together.
Example
Consider the problem:
100.37
2.356
+ 6000.652274
Two places after the decimal
Three places after the decimal
Six places after the decimal
Example
Consider the problem:
100.37
2.356
+ 6000.652274
6103.37
Two places after the decimal
Three places after the decimal
Six places after the decimal
Two places after the decimal in
the answer.
Multiplication and Division
• The answer matches the smallest number of
significant digits as the numbers you are
multiplying or dividing together.
Example
• Consider the problem:
120
x
2.28
273.6
Two significant digits
Three significant digits
Needs Two significant digits
Example
• Consider the problem:
120
x
2.28
270
This number gets rounded
off and the last digit
becomes a 0
Two significant digits
Three significant digits
Two significant digits
Practice
• Complete the worksheet on Significant Figures
for your classwork today.