Grade 3: Unit Conversions

Warm up with a few quick unit conversions warm-ups on the board while the class settles, so every child starts thinking about the skill.

Converting length means writing the same distance in a different unit, such as changing metres to centimetres. In the metric system the length units mm, cm, m and km scale by powers of ten, so every conversion is a multiply or divide by 10, 100 or 1000. Knowing the size of each unit and which way to convert is the whole skill. Model the method clearly, thinking aloud:

• Anchor each unit to something real: a millimetre is about the thickness of a coin, a centimetre a fingernail's width, a metre a big stride, a kilometre a short walk.
• Learn the key relationships: 10 mm in a cm, 100 cm in a m, 1000 m in a km.
• Decide the direction: converting to a smaller unit multiplies (the number gets bigger), converting to a larger unit divides (the number gets smaller).
• Sense-check the size: 2 m written in centimetres should be a bigger number (200 cm), not a smaller one.
• Progress to comparing and ordering lengths given in mixed units by converting them to the same unit first.

Work this through step by step on the board, then have the class talk you through a second one.

• Convert 250 cm to metres:
• 100 cm = 1 m
• a larger unit, so divide
• 250 / 100 = 2.5 m

Do the first few questions of the practice worksheet together, one child explaining each step. Check for understanding before releasing the class to work alone.

Students complete the practice worksheet independently while you circulate and support.

Circulate and look for these, they are the usual sticking points:

• Multiplying when you should divide, so the answer is the wrong size.
• Using the wrong relationship (100 cm in a metre, not 10).
• Moving the decimal point the wrong number of places.
• Dropping the unit off the answer, so 250 becomes meaningless.

Pull the class back together. Ask one child to explain unit conversions in their own words, pose a single check question everyone answers on a mini whiteboard, and name what you will build on next lesson.

Use the independent worksheet as the evidence. A child who can complete it accurately and explain one answer has met the objective; anyone who cannot needs the easier level and a short reteach next session.