Primary 3 Length, Mass and Volume Study Guide

Primary 3 length, mass and volume in four short steps, from measuring a ribbon to adding litres and millilitres. Each step has something to tap and a question to try.

Start here

What are compound units?

One measurement, written in a big unit and a small unit.

A ribbon that is 2 m 46 cm long is 2 whole metres and 46 more centimetres. Since 1 m is 100 cm, the same ribbon is also 246 cm long. Primary 3 uses four pairs of units: km and m, m and cm, kg and g, and litres and millilitres.

In this chapter, your child learns to:

  • choose a sensible unit, and measure a length longer than the ruler;
  • read a weighing scale or a measuring jug by finding what one small gap stands for;
  • change a measurement in two units into the smaller unit, and back again;
  • change to the same unit before adding or subtracting, then give the unit the question asks for.
1

Step 1

Choosing units of measurement

Pick the unit that fits the size, then measure. Long distances, such as a bus ride, are in km; a ribbon is in m and cm; a bag of rice is in kg and a pencil case in g; a flask holds litres and a cup holds ml. When a ribbon is longer than the ruler, make a pencil mark where the ruler ends, slide the ruler along to start at the mark, and count the whole metres. Then write the extra centimetres.

Tap Mark and slide

1 m1 m05010046 cm? m ? cm
The ribbon is longer than the 1 m ruler. Tap Mark and slide.
Try it

A school bus travels about 12 ___ from Woodlands to the city centre. Which unit fits?

Choose and measure units: 1 lesson, 8 questionsChoose and measure units: 1 lesson, 8 questions
2

Step 2

Reading scales and measuring jugs

Find what one small gap stands for first. Look at the two numbered marks on either side of the pointer or the water level. Count the equal gaps between them and divide to find one gap. Then count up the gaps from the lower numbered mark.

Tap Next

0 ml100 ml200 ml300 ml400 ml5 gaps1 gap = 20 ml
The water level is between 200 ml and 300 ml. Tap Next to read it.
Try it

A jug is marked 0, 100 ml and 200 ml, with 4 equal gaps between each pair of numbered marks. The water level is 3 gaps above 100 ml. How much water is in the jug?

Read scales and levels: 1 lesson, 8 questionsRead scales and levels: 1 lesson, 11 questions
3

Step 3

Converting compound units

Change the whole big units; the extra small units stay as they are. Use 1 km = 1000 m, 1 m = 100 cm, 1 kg = 1000 g and 1 ℓ = 1000 ml. To write 2 kg 45 g in grams, two kilograms make two thousand grams, and the 45 g stays 45 g. To go back, take out the whole big units and leave the rest as small units.

Pick a measurement, then tap the button

2 kg 45 g1 kg1 kg45 g
    2 kg 45 g is 2 whole kilograms and 45 extra grams.
    Try it

    Write 4 kg 8 g in grams.

    Convert measurements: 1 lesson, 8 questionsConvert measurements: 1 lesson, 9 questions
    4

    Step 4

    Length, mass and volume word problems

    Change to the same unit before you add or subtract. Write both amounts in the smaller unit, work out the total or what is left, then change the answer back into the units the question asks for.

    Tap Next step

    A jug has 1 ℓ 300 ml of orange juice. 900 ml more is poured in. How much juice is in the jug now?

    1 ℓ 300 ml900 ml?
      The two parts are in different units. Change them to the same unit first.
      Try it

      A path is 1 km 200 m long. Ravi has walked 450 m of it. How much farther must he walk?

      Convert and calculate: 1 lesson, 8 questionsConvert and calculate: 1 lesson, 8 questionsMixed practice: 12 questions

      Primary 3 length, mass and volume questions, worked step by stepHave a go on paper first. Then open the hint, or show the working one line at a time.

      Choosing and measuringBasic

      Mei Ling measures a curtain with a 1 m ruler. The curtain is 2 rulers long and 35 cm more. Write its length in m and cm, and then in cm.

      Each full ruler is 1 m. Then change the whole metres to centimetres.
      1. 2 rulers = 2 m
      2. Length = 2 m 35 cm
      3. 2 m = 200 cm
      4. 200 cm + 35 cm = 235 cm
      2 m 35 cm, or 235 cm
      Reading scalesBasic

      A measuring jug is marked 0, 500 ml and 1 ℓ. There are 5 equal gaps between each pair of numbered marks. The water level is 2 gaps above the 500 ml mark. How much water is in the jug?

      1 ℓ is 1000 ml, so the marks go up 500 ml at a time. Find what one gap stands for.
      1. From 500 ml to 1000 ml is 500 ml
      2. 500 ÷ 5 = 100, so 1 gap = 100 ml
      3. 2 gaps = 200 ml
      4. 500 ml + 200 ml = 700 ml
      700 ml
      ConvertingIntermediate

      A box of books has a mass of 3065 g. Write its mass in kg and g.

      Take out the whole kilograms. 1000 g make 1 kg.
      1. 3065 g = 3000 g + 65 g
      2. 3000 g = 3 kg
      3. 3065 g = 3 kg 65 g
      3 kg 65 g
      Convert and calculateIntermediate

      A tank has 4 ℓ 250 ml of water. Siti pours in another 1 ℓ 900 ml. How much water is in the tank now? Give your answer in ℓ and ml.

      Change both amounts to millilitres before adding.
      1. 4 ℓ 250 ml = 4250 ml
      2. 1 ℓ 900 ml = 1900 ml
      3. 4250 ml + 1900 ml = 6150 ml
      4. 6150 ml = 6 ℓ 150 ml
      6 ℓ 150 ml
      Convert and calculateChallenge

      Hui Min has 5 m of ribbon. She cuts 4 pieces, each 85 cm long. How much ribbon is left? Give your answer in m and cm.

      Change 5 m to centimetres. Then find how much the 4 pieces use.
      1. 5 m = 500 cm
      2. 4 × 85 cm = 340 cm
      3. 500 cm − 340 cm = 160 cm
      4. 160 cm = 1 m 60 cm
      1 m 60 cm

      Check yourself

      Check yourself on length, mass and volume

      Five quick questions on units, scales and converting. Each one points to the subtopic to practise next.

      A packet of rice has a mass of about 5 ___. Which unit fits?

      A scale is marked 0 kg, 1 kg and 2 kg, with 4 equal gaps between each pair of numbered marks. The pointer is 3 gaps past 1 kg. What mass does it show?

      Write 2 km 50 m in metres.

      Write 506 cm in m and cm.

      A bottle has 1 ℓ 500 ml of water. Amir drinks 700 ml. How much water is left?

      0/5
      Answer all five to see where to practise next.
      For parents

      Measure things together at home. Let your child measure a table with a 1 m ruler or tape, weigh fruit on a kitchen scale, and pour water into a measuring jug. Ask them to say each reading in two units, such as 1 m 20 cm, and then in the smaller unit.

      When a reading falls between numbered marks, ask "What does one small gap stand for?" before your child counts. For word problems, ask which unit both amounts should be in before any adding or subtracting. Then try the worked examples on paper and reveal the lines one at a time.

      More study guides

      Before this: FractionsNext: Area and PerimeterPrimary 3 Time study guidePrimary 3 Money study guidePrimary 3 Addition and Subtraction Within 10 000 study guide
      Lessons and practice

      P3 Length, Mass and Volume lessons and practice

      4 lesson videos and 48 questions across four subtopics, from Basic to Challenge, each with a worked solution.

      Watch and practise →