Addition and Subtraction of Units of Lengths


Lesson Planning of Addition and Subtraction of Units of Lengths

Subject Mathematics

Grade 4th

Students` Learning Outcomes

  • Add and subtract expressions involving similar units of lengths.
  • Use appropriate units to measure lengths of different objects.
  • Solve real life problems involving conversions, addition and subtraction of units of lengths.
  • Information for Teachers
  • Milometer, meter, centimeter and millimetre are units of measurement written as km, m, cm and mm respectively.
  • Conversion of units can be done by using the following rules, as;

1 KM = 1000 M

1 M = 10 CM

1 CM = 10 MM

  • With the help of these units of length we can find required measurements and can also change them into appropriate measure.
  • We multiply while converting greater to smaller unit and use division while converting small unit to greater.
  • During this lesson consult text book at each lever wherein it is required

Material / Resources

Writing board, chalk/marker, scale, measuring tape, textbook


  • Write on the board kilometres, meters, centimetres and millimetres.
  • Ask the students which unit can be used for measuring the following: as;
  • Distance between cities (Milometers)
  • Length of a pencil (Centimetre)
  • The thickness of finger nails (Millimetres)
  • Tell them that we have learnt basic units of length in previous lectures.
  • Today we will learn how to add and subtract these units.
  • Ask them can we add millimetre into kilometre? (Expected answer would be as; No)
  • In fact, only similar units can be added or subtracted
  • We make different units similar before adding and subtracting.


Activity 1

  • Ask the class how many units can you see written on a scale? (Expected answer would be as; inch, centimeter and millimetre)
  • Observe how many centimetres does it contain? (Expected answer would be as; 30 cm)
  • If I have 4 such scale how many centimetres they will make altogether?
  • 1 Scale = 30 cm
  • 4 Scale = 30 cm + 30 cm + 30 cm = 120 cm.


Activity 2

  • “Zenith went to a tailor and gives him 4 meters cloth to stitch her shirt. Tailor returned 1 meter cloth to her after stitching shirt. How much cloth is used in her shirt?”
  • Solve on the board with the help of subtraction.
  • Ask them do you all wear same size of uniform? (Expected answer would be as; No)
  • Tell them that usually the distance is measured in kilometres, whereas the length are measured in meters and centimetres and even very small lengths are measured in millimetres.
  • Give examples: as;
  • Distance from Lahore to Karachi = km
  • The length of a Bundle of cloth = m
  • The length of a needle to stitches – mm
  • Call the name of different things from the classroom and ask the students at random which unit they will use to measure each one of them.



Sum up / Conclusion
  • Same units can be added or subtracted only.
  • If we want to add them we first convert them in same units.
  • The smallest unit for measuring length is millimetre.
  • The greatest unit for measuring length, height or distance is kilometers.
  • We multiply while converting greater to smaller unit and use division when we convert smaller to greater unit.


  • Give following problems to the students to assess them: as;
  • Distance between cities A to B is 450 kilometres. If the driver has covered 291 kilometres, then how many kilometres are left to be covered?


  • Choose the appropriate units for measuring the following lengths: as;
  • Length of shoes:

(a                   (a) Meter  (b) centimetres  (c) millimetres

  • Width of an eraser:

(a                 (a)  Millimetre  (b) meters   (c) kilometres

  • Assign following questions to the students: as;

Follow up

  • Choose the best estimate to answer each question: as;
  • About how tall is a plastic water bottle?

                       (a) 20 cm   (b)  20 km

                        © 20 m    (d) 20 mm

  • About how long a pens nib is?

(a                        (a)   2 cm   (b)  2 mm

                 (c)  2 m     (d) 2 km

  • About how away from can be my school from my home?

(a                           (a) 1 m    (b)  1 km

                    © 1 cm   (d) mm







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