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Maths

Converting Units of Length, Mass and Capacity

Overview

Using physical measurement contexts and known conversions, students practise converting within the metric system and use these conversions to compare and order measurements.

Learning Objective
Students convert between standard metric units of length (m/cm/mm), mass (kg/g), and capacity (l/ml) and apply these conversions in measurement problems.

Resources needed

  • Rulers
  • Simple scales or weight labels
  • Measuring jugs (optional)
  • Conversion reference on board

Lesson stages

0 / 7 done
  1. 1 Display: 1m = 100cm = 1000mm. 1kg = 1000g. 1 litre = 1000ml. 'Metric units are based on powers of 10 — which makes converting much easier than older systems.' Ask: 'Why is 1kg = 1000g, not 100g?'
  2. 2 Convert 2.4m to cm: × 100. Answer: 240cm. Convert 3.5kg to g: × 1000. Answer: 3500g. Students convert six measurements from larger to smaller units. Key: multiplying makes the number bigger — because smaller units mean you need more of them.
  3. 3 Convert 350cm to m: ÷ 100. Answer: 3.5m. Convert 2750g to kg: ÷ 1000. Answer: 2.75kg. Students convert six measurements, including some with remainders (e.g. 245cm = 2.45m).
  4. 4 Which is heavier: 1.5kg or 1450g? Convert to the same unit: 1.5kg = 1500g > 1450g. Students compare six pairs of measurements in different units by converting to a common unit first.
  5. 5 A recipe needs 750ml of water. A jug holds 0.5 litres. How many full jugs are needed? (0.5l = 500ml; 750 ÷ 500 = 1.5, so 2 jugs.) Students solve three similar problems.
  6. 6 Students measure classroom objects (desk width, pencil length, book height) in cm. Convert each to mm and to m. Record in a table.
  7. 7 Convert: 3.7m = ___ cm. 4200g = ___ kg. 850ml = ___ litres. Which is more: 1.3kg or 1250g?

Tap a step to mark it as done.

Variations

  • Extend to converting between km and m (÷/× 1000)
  • Include perimeter problems with mixed units
  • Compare metric and imperial (rough conversions: 1kg ≈ 2.2 pounds)
More information

Display all conversions and the rule: 'to convert to smaller units, multiply; to convert to larger units, divide.' Teach prefixes: milli = 1/1000, centi = 1/100, kilo = 1000×.

Provide a conversion reference card. Focus on one type of measurement at a time. Use visual number lines for conversions.

Do students multiply (not divide) when converting to smaller units? Do they convert to the same unit before comparing? Do they handle decimal conversions correctly?

Rulers for length measurement. All other conversions done on paper. No scales or jugs needed.

Students may multiply when converting to larger units (e.g. writing 2000cm when converting 2kg to grams). Reinforce: 'going to smaller units → more of them → multiply.'

Activity details
Primary Medium 30 minutes 5 minutes prep Solo, Pairs Indoor
Topics
Measurement Conversion Length Mass Capacity