Grade by Grade
B. Number · Number Sense: Numbers to 10 000, Fractions and Decimal Tenths

Rounding Decimal Tenths to the Nearest Whole Number

B1.8About 30 minutes
Learning goals
  • I can round a decimal tenth to the nearest whole number.
  • I can use a number line to find the closer whole number.
  • I can round a decimal that is exactly halfway.
  • I can use rounding in real-life situations.

Words to know

round
Change a number to a nearby friendly number. Here, the closest whole number.
nearest whole number
The whole number that is closest, like 4 for 3.7.
halfway
Exactly in the middle. 3.5 is halfway between 3 and 4.

Let's start with a story

The weather report says it is 21.7 °C today. Grandpa asks, "About how warm is it?"

21.7 is between 21 and 22. It is closer to 22. So we can say it is about 22 °C. We rounded 21.7 to the nearest whole number.

2221.5 (halfway)2121.7 °C3 tenths to 227 tenths to 21
A close-up of a thermometer between 21 °C and 22 °C. 21.7 is only 3 tenths from 22 but 7 tenths from 21, so it rounds to 22 °C.

How to round a decimal tenth

  1. Find the two whole numbers the decimal is between. (3.7 is between 3 and 4.)
  2. The halfway point is .5 (3.5).
  3. Look at the tenths digit:
  • 0, 1, 2, 3 or 4: round down to the smaller whole number (keep the ones digit).
  • 5, 6, 7, 8 or 9: round up to the next whole number.

This is the same rule you used for whole numbers: when it's exactly halfway, round up.

33.54
3 to 4 by tenths. 3.2 is closer to 3, so it rounds to 3. 3.7 is past halfway (3.5), so it rounds to 4.
Worked example

Example 1: Round down

Round 6.3 to the nearest whole number.

  1. 6.3 is between 6 and 7.
  2. The tenths digit is 3, which is less than 5.
  3. 6.3 is closer to 6.

6

Worked example

Example 2: Exactly halfway

Round 12.5 to the nearest whole number.

  1. 12.5 is between 12 and 13.
  2. It is exactly halfway.
  3. When it's halfway, we round up.

13

Worked example

Example 3: Rounding up to the next ten

Round 9.6 to the nearest whole number.

  1. 9.6 is between 9 and 10.
  2. The tenths digit is 6, so round up.

10

Worked example

Example 4: Less than 1

Round 0.4 and 0.8 to the nearest whole number.

  1. Both are between 0 and 1.
  2. 0.4: the tenths digit is 4, so round down to 0.
  3. 0.8: the tenths digit is 8, so round up to 1.

0.4 rounds to 0. 0.8 rounds to 1.

SituationExactRounded
Temperature21.7 °Cabout 22 °C
Race time18.4 secondsabout 18 seconds
Snowfall4.5 cmabout 5 cm
Ski trail2.3 kmabout 2 km
Rounding decimals in real life.

Quick check

After you round, check with "Is my answer one of the two whole numbers the decimal is between?" If you round 6.3 and get 7.3 or 63, something went wrong!

Just for fun

Grandpa's weather report: "It's 21.7 °C. That rounds to 22 °C, which rounds to shorts weather!"

Hands-on activity

Round the measurements

You need: a tape measure or ruler, a few objects, a thermometer if you have one, paper and a pencil.

  1. Measure 5 objects in centimetres, to the nearest tenth of a centimetre if you can (each tiny line between centimetres on a ruler is a millimetre, which is 0.1 cm). For example, a crayon might be 8.6 cm long.
  2. Write each length as a decimal.
  3. Round each one to the nearest whole centimetre.
  4. Use a number line to show your grown-up why one of your answers is correct.

Math talk: "When would you want the exact length and when is the rounded length good enough?"

Your turn

1

Round 4.2 to the nearest whole number.

2

Round 4.8 to the nearest whole number.

3

Round 7.5 to the nearest whole number.

4

Round 0.6 to the nearest whole number.

5

Round 19.7 to the nearest whole number.

6

A pumpkin has a mass of 3.4 kg. About how many kilograms is it?

More practice: warm-up

1

Round 2.1.

2

Round 2.9.

3

Round 5.5.

4

What two whole numbers is 8.3 between?

5

What is halfway between 6 and 7?

More practice: on your own

1

Round 15.4 and 15.6 to the nearest whole number.

2

Round 0.5 to the nearest whole number.

3

A hike is 6.8 km. About how long is it, to the nearest kilometre?

4

Write two decimals that round to 4.

5

Round 29.5 to the nearest whole number.

Challenge

1

What is the least and the greatest decimal tenth that rounds to 7?

2

Two ribbons are 1.5 m and 1.4 m long. Both are rounded to the nearest metre. Are the rounded lengths the same? Explain.

3

Tia rounds 8.4 and gets 9. Explain her mistake.

Check your understanding

Quiz

1.

Round 5.3 to the nearest whole number.

2.

Round 2.5 to the nearest whole number.

3.

Round 9.7 to the nearest whole number.

4.

Which decimal rounds to 6?

5.

On a summer day it is 24.6 °C. About how warm is it?

6.

Round 0.3 to the nearest whole number.

For parents and teachers

Big idea: B1.8 asks Grade 4 students to round decimal numbers to the nearest whole number in various contexts. Rounding helps children estimate and make sense of measurements like temperatures and distances.

Common difficulties:

  • Not ending with a whole number (rounding 8.4 to 8.5 or 9.4). The answer must be one of the two whole numbers the decimal is between: 8 or 9.
  • Rounding up at 0.4 or down at 0.5. Use a number line from one whole to the next with the halfway point marked.
  • Crossing a ten (9.6 → 10, 19.7 → 20).

How to help: Use thermometers, weather reports, kitchen scales and car trip distances. Ask "About how much is that, to the nearest whole number?"