NCERT Class 4 Maths · Chapter 6

Measuring Length

Metres and centimetres: estimating, measuring with a scale or tape, converting m ↔ cm, and perimeter (the length around a shape).

📌 What you’ll learn in this chapter
  • Length, height, breadth, and depth are all lengths
  • Using 1 m, ½ m, and ¼ m ropes
  • The centimetre: 1 m = 100 cm
  • Measuring correctly with a scale (even a broken one!)
  • Perimeter: the length of a boundary

🏃 Let’s Begin: Sports Day Lines

PT Sir needs a 10 m line for the running race. He has only a 1 m rope. How can he make it? He lays the rope down 10 times, end to end, marking with chalk each time.

Now the children know how long 10 m feels, about 12–14 big steps for a child!

📏 Part 1: The Metre

  • A door is about 2 m tall, and a Class 4 child is a little more than 1 m tall.
  • ½ m + ½ m = 1 m · ¼ m + ¼ m + ¼ m + ¼ m = 1 m · ¼ m + ¼ m = ½ m
💡 Body measures (to estimate) From your nose to your outstretched fingertip is about ½ m for a child. One big step is about ½ m. Use these to guess, then check with a tape.

🐜 Part 2: Centimetres

A metre is too big to measure an eraser or a seed. So we divide 1 m into 100 equal parts. Each part is 1 centimetre (cm).

1 m = 100 cm  ·  ½ m = 50 cm  ·  ¼ m = 25 cm  ·  ¾ m = 75 cm

About 1 cm: the width of your little fingernail, a grain of rice (a bit less), an ant.

How to measure with a scale

  1. Put the 0 mark (not the edge of the scale!) at one end of the object.
  2. Keep the scale straight along the object.
  3. Read the number at the other end.

Broken scale? No problem! If a pencil starts at 3 cm and ends at 12 cm, its length is 12 − 3 = 9 cm.

⚠️ Common mistake Starting from 1 instead of 0 makes every answer 1 cm too big. ✅ Start at 0, or subtract the starting number.

🔁 Part 3: Metres and Centimetres Together

Riya’s height is 132 cm. We can also say 1 m 32 cm. Both are correct!

  • 3 m = 300 cm · 2 m 45 cm = 200 + 45 = 245 cm
  • 560 cm = 500 cm + 60 cm = 5 m 60 cm
  • 1 m 5 cm = 105 cm (not 15 cm!)

🧱 Part 4: Perimeter

The total length around the edge of a shape is its perimeter. Think of the fence around a garden, or the lace around a handkerchief.

A rangoli board has sides of 30 cm, 20 cm, 30 cm, and 20 cm. Perimeter = 30 + 20 + 30 + 20 = 100 cm = 1 m.

On a dot grid where each gap is 1 cm, just count the gaps along the boundary.

Different shapes can have the same perimeter: a 3 × 2 rectangle and a 4 × 1 rectangle both have a perimeter of 10 cm.

✏️ Practice Time

Try these on your own first, then check your answer below each question.

Q1. Would you measure a bedsheet in cm or m? A pencil?
✅ AnswerBedsheet: m · Pencil: cm
Q2. 4 m = ___ cm, and 350 cm = ___ m ___ cm
✅ Answer400 cm and 3 m 50 cm
Q3. A crayon starts at 2 cm and ends at 9 cm on a scale. How long is it?
✅ Answer9 − 2 = 7 cm
Q4. A square card has 8 cm sides. What is its perimeter?
✅ Answer8 + 8 + 8 + 8 = 32 cm
Q5. How many 25 cm sticks make 1 m?
✅ Answer4

📘 NCERT Textbook Activity Help

Hints and answers for the activities in the textbook, page by page. Try each one yourself before looking at the answer!

Page 80 · Let Us Observe

What is measured?

✅ AnswerThey are measuring length, height, breadth, and depth. Weight and temperature are not lengths. A measuring tape (or a height chart on the wall) is used for height. A metre scale and a stadiometer can also be used.
For most classrooms: (a) correct (b) correct (c) incorrect, a door is about 2 m (d) correct. Check in your own classroom.
Pages 81–82 · Ropes, long jump, classroom

Measuring with ropes

✅ AnswerMake a 5 m line by laying the 1 m rope 5 times, and a 10 m line by laying it 10 times (or the 5 m line twice). Long jump and classroom size depend on your class. Estimate first, then measure with rope combinations, e.g. 1 m + ½ m + ¼ m = 1¾ m.
Page 83 · Guess the Length

Whale, bus, giraffe, and crocodile

✅ Answer1. Two buses = 30 m, so one bus = 15 m. The crocodile (6 m) is shown as 6 bats, so one cricket bat is about 1 m. 2. The whale is as long as 2 buses, so two whales = 4 buses. 3. 30 cricket bats. 4. 3 m + 3 m = 6 m, which is more than the 4 m tub, so two ostriches are taller than the tub (the giraffe is about 6 m tall). 5. 30 ÷ 6 = 5 crocodiles.
Page 84 · Centimetres

Parts of a metre

✅ AnswerThe tailor’s tape and the metre rope can be the same length (tailor’s tapes are often 150 cm long). The tape has number marks for every cm. ½ m = 50 cm, and ¼ m = 25 cm.
Pages 85–88 · Let Us Do

Measuring small things

✅ AnswerMeasure the pictures in your book with a scale, starting at 0. A fingernail and an ant are about 1 cm or less. An eraser is more than 1 cm. Wheat and rice grains are less than 1 cm. A rajma seed is about 1–1.5 cm.
Treasure hunt: count the 1 cm tiles along each route. The route with the fewest tiles is the shortest.
Broken scale (Q7): subtract the starting mark from the ending mark. The blue paper runs from about 8 to 14 cm, so it is about 6 cm. Check the patterned paper the same way.
Page 89 · Number lines and stickers

Fill the blanks

✅ AnswerThe first line needs 50 cm and 75 cm. The second line needs ¼ m (= 25 cm) and ½ m.
Stickers: 2 m = 200 cm, and 200 ÷ 20 = 10 stickers.
Ramu and Shamu are both correct: 120 cm = 1 m 20 cm.
Page 90 · Wells

Same depth

✅ Answer(ii) 4 m (iii) 600 cm (iv) 8 m = 800 cm
1 m 40 cm = 140 cm · 4 m 60 cm = 460 cm · 2 m 30 cm = 230 cm · 5 m 50 cm = 550 cm
Pages 91–94 · Perimeter

Bricks, gardens, and dot grids

✅ AnswerBricks: count only the bricks on the outer edge of the garden.
Q1: The garden with the most “corners in and out” (the cross shape) usually has the longest boundary. Count the bricks to check.
Q2: Count the 1 cm gaps around each shape. The shape that goes in and out the most has the longest boundary, and the small square (2 × 2 = 8 cm) has the shortest.
Q3: Counting the gaps: A = 12 cm · B = 10 cm · C = 10 cm · D = 14 cm · E = 14 cm · F = 14 cm. So B and C match, and D, E, and F match. (Cutting a square from a corner doesn’t change the perimeter!)
Q4: The plain rectangle (first garden) has the smallest perimeter. The garden with a notch cut into the middle of its top has a longer one.
Q6: Shapes with a 20 cm perimeter, e.g. rectangles 6 × 4, 7 × 3, 8 × 2, or a 5 × 5 square.

👨‍👩‍👧 Tips for Parents

  • Measure family members’ heights and write them in cm and in m cm.
  • Let your child measure small things with a ruler, always starting at 0.
  • Find the perimeter of a table using a tape, adding all four sides.
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