Measurement
What you must understand
This domain covers length, mass, capacity, time, money, perimeter, area and volume. Candidates must know the metric relationships: 10 mm = 1 cm, 100 cm = 1 m, 1000 m = 1 km, 1000 g = 1 kg, 1000 ml = 1 litre. Time needs 60 seconds = 1 minute, 60 minutes = 1 hour, 24 hours = 1 day, and the number of days in each month. You should calculate perimeter of rectangles and simple compound shapes, area of rectangles, triangles and squares, and volume of cuboids. Money calculations must keep two decimal places.
Traps to watch
- Mixed units: a question may give length in metres and width in centimetres. Convert before calculating.
- Perimeter versus area: perimeter is a length (cm, m); area is in square units (cm², m²). Never label area in cm.
- Triangle area: remember to halve base × height, and use the perpendicular height, not a slanted side.
- Time across the hour or midnight: count minutes carefully; for example, 11:45 to 12:20 is 35 minutes, not 25.
- Capacity versus volume: 1 cm³ holds 1 ml, and 1000 cm³ holds 1 litre. Convert before comparing.
Worked example
A rectangular tank is 50 cm long, 30 cm wide and 20 cm high. How many litres does it hold when full?
Volume = 50 × 30 × 20 = 30 000 cm³. Since 1000 cm³ = 1 litre, capacity = 30 000 ÷ 1000 = 30 litres.
Rule of thumb: write the unit beside every number as you work; if two units differ, convert to the smaller unit first.