Decimals

UK Secondary (7-11)

Round numbers with decimals

Lesson

When we round a number to the nearest thousandth, we first want to know about its decimal values. A number like $0.0537$0.0537 can be written in a place value table as shown below.

Units | . | Tenths | Hundredths | Thousandths | Ten-thousandths |
---|---|---|---|---|---|

$0$0 | $.$. | $0$0 | $5$5 | $3$3 | $7$7 |

We can see that the thousandths is the third decimal place.

To round this number, we look at the ten thousandths, or the fourth decimal place. If the place value is less than $5$5 we round down, and if it's greater or equal to $5$5 we round up.

The number $0.0537$0.0537 lies between $0.053$0.053 and $0.054$0.054, and because the place value in the fourth decimal place is $7$7, we round **up** to $0.054$0.054.

We can round to the nearest ten-thousandth, or to four decimal places, using the same procedure.

Round $45.43021$45.43021 to four decimal places.

**Think:** We want to look at the place value in the fifth decimal place.

**Do:** The place value in the fifth decimal place is $1$1. So we round the number $45.43021$45.43021 **down** to $45.4302$45.4302.

Round $45.09997$45.09997 to four decimal places.

**Think:** We want to look at the place value in the fifth decimal place.

**Do:** The place value in the fifth decimal place is $7$7. So we round the number $45.09997$45.09997 **up**.

Ordinarily this means we increase the place value by $1$1, but whenever there is a $9$9, we change it to zero $0$0 and add the $1$1 to the place value to the left.

So the number $45.09997$45.09997 rounds **up** to $45.1000$45.1000.

Remember!

To round a decimal value to a certain number of decimal places, look at the next decimal place value to the right.

If it's less than $5$5 then round down and if it's greater than or equal to $5$5 then round up.

Round $430.4875$430.4875 to the nearest thousandth.

Round $7.034500$7.034500 to four decimal places.

Round $99.097906$99.097906 to four decimal places.