How do you round numbers to 3 significant figures?

Answer 1

There are three rules that are used when rounding to a desired number of significant digits (figures):

1. All digits that are not zero, are significant

2. In a number that does not have a decimal point, all zeros between two non-zero digits are significant digits

3. In a number that has a decimal point, all zeros after the leftmost non-zero digit are significant

Examples:

12345 rounded to 3 significant digits: 12300, or 1.23 x 104

12.345 rounded to 3 significant digits: 12.3, or 1.23 x 101

0.012345 rounded to 3 significant digits: 0.0123, or 1.23 x 10-2

0.012045 rounded to 3 significant digits: 0.0120, or 1.20 x 10-2

In the last example the zero after 2 is significant. That is the reason for keeping it in the result when rewriting it in powers of 10 notation.

Answer 2

To round numbers to 3 significant figures, identify the first three non-zero digits from the left. If the digit after the third significant figure is 5 or more, round up the last significant figure. If the digit after the third significant figure is less than 5, keep the last significant figure the same. Trailing zeroes can be kept to maintain the significant figures.