Estimating measures

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By John Spooner teacher and freelance writer

Make cross-curricular links to numeracy with these measurement activities for ages 7-11

Estimates of measures are educated guesses based on comparisons with quantities that we already ‘know’. For example, we might estimate the width of a book using a mental image of a 30cm ruler, the length of a room using rounded multiples of our own height, or the volume of water in a fi sh tank using an image of a litre carton of fruit juice. In order to develop their estimation skills, children need to explore, develop and internalise a number of their own mental images or ‘yardsticks’. They also need lots of practice, so that they can develop their spatial skills and improve upon their accuracy and consistency.

Talking points:

  • What sort of mental images or techniques could you use when estimating counting seconds, for example ‘one Mississippi, two Mississippi…’?
  • Does the degree of accuracy required for a ‘good’ estimate remain constant?
  • What’s an acceptable range for an estimate of the width of this window or mass of this book or length of this pause?
  • When would it be appropriate to estimate rather than measure?

Children should be encouraged to give a range (for example, 1.5-2kg) rather than a precise figure (for example, 2.4kg). They need to recognise that accurate estimation not only requires the measurement to fall within the range given in their estimate, but also involves setting a ‘suitable’ range that is as small as is appropriate. For example, a range of 2cm is appropriate for an estimation of the length of a pencil, but not for an estimation of the length of a car. Children will also need to think about the circumstances where measurement should be used in place of estimation.

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