Rounding means making a number simpler but keeping its value close to what it was.

The result is less accurate, but easier to use.

There are many ways to round. This is the most common method:

• Decide which is the last digit to keep.

• Leave it the same if the next digit is less than 5 (this is called rounding down)

• Increase it by 1 if the next digit is 5 or more (this is called rounding up) 

Significant figures are the number of digits that are meaningful: they have an accuracy matching our measurements, or are simply all we need.

RULES FOR SIGNIFICANT FIGURES

1. All non-zero numbers ARE significant. The number 33.2 has THREE significant figures because all of the digits present are non-zero.

2. Zeros between two non-zero digits ARE significant. 2051 has FOUR significant figures. The zero is between a 2 and a 5.

3. Leading zeros are NOT significant. They're nothing more than "place holders." The number 0.54 has only TWO significant figures. 0.0032 also has TWO significant figures. All of the zeros are leading.

4. Trailing zeros to the right of the decimal ARE significant. There are FOUR significant figures in 92.00.

92.00 is different from 92: a scientist who measures 92.00 milliliters knows his value to the nearest 1/100th milliliter; meanwhile his colleague who measured 92 milliliters only knows his value to the nearest 1 milliliter. It's important to understand that "zero" does not mean "nothing." Zero denotes actual information, just like any other number. You cannot tag on zeros that aren't certain to belong there.

5. Trailing zeros in a whole number with no decimal shown are NOT significant. Writing just "540" indicates that the zero is NOT significant, and there are only TWO significant figures in this value.

6. Exact numbers have an INFINITE number of significant figures. This rule applies to numbers that are definitions. For example, 1 meter = 1.00 meters = 1.0000 meters =1.0000000000000000000 meters, etc.

Exercise: Round 1000.3 to four significant figures. 1000.3 has five significant figures (the zeros are between non-zero digits 1 and 3, so by rule 2 above, they are significant.) We need to drop the final 3, and since 3 < 5, we leave the last zero alone. so 1000. is our four-significant-figure answer. (from rules 5 and 6, we see that in order for the trailing zeros to "count" as significant, they must be followed by a decimal. Writing just "1000" would give us only one significant figure.)