Measurement
What you need to understand
Measurement questions ask you to choose appropriate units, convert between them, and calculate lengths, perimeters, areas, volumes and times. You should know the metric prefixes (milli-, centi-, kilo-) and be able to move confidently between millimetres, centimetres, metres and kilometres, and between millilitres, litres and cubic units. You also need to read clocks and timetables, calculate elapsed time, and work with 12- and 24-hour time. Perimeter, area and volume formulas for rectangles, triangles and simple prisms should be familiar, and you should understand why area uses square units and volume uses cubic units.
Traps to watch for
- Squared and cubed conversions. 1 m = 100 cm, but 1 m² = 10,000 cm² and 1 m³ = 1,000,000 cm³.
- Elapsed time across the hour or midnight. Count in stages rather than subtracting digits.
- Perimeter versus area. Read the question carefully to see which is asked.
- Units in the answer. Always state the unit, and check it matches the question.
Worked example
A rectangular garden is 8 m long and 5 m wide. A path 1 m wide runs around the outside. What is the area of the path?
Outer rectangle: 10 m × 7 m = 70 m². Inner garden: 8 m × 5 m = 40 m². Path area = 70 − 40 = 30 m². The trap is to add the path width to only one side of each dimension; it must be added to both ends.
Rule of thumb
Sketch the shape, label every dimension in the same unit, then apply the formula.