Understand the idea.
A thoughtful guess
An estimate is a sensible guess supported by something you know. Compare a length with a familiar ruler, or a capacity with a litre bottle. Ask whether the object is shorter, longer, smaller or larger than that benchmark.
An estimate need not be exact. It should be close enough to help you notice a surprising result. A book-sized object is unlikely to be only a few millimetres long or several metres long.
Estimate, measure, compare
Estimate first and include a unit. Then measure with a suitable tool and compare. Keeping the estimate separate helps you see how well your chosen benchmark worked.
Use the same unit when finding the difference. Subtract the smaller measurement from the larger one. Then say whether your estimate was too high or too low; the difference alone does not tell that direction.
See the method in action.
A ribbon
A ribbon looks a little longer than a 30 cm ruler. Choose 35 cm or 3 cm as an estimate.
Worked solution
- 1
Use the ruler as the benchmark.
The ribbon is described as a little longer than this known length.
- 2
Test the 3 cm choice.
Three centimetres is shorter than the ruler, so it contradicts the description.
- 3
Test the 35 cm choice.
Thirty-five centimetres is only a little longer than the 30 cm benchmark, so it is the sensible estimate.
35 cm.
A bag
Sam estimates a bag at 2 kg. The scale shows 3 kg. How far apart are the values?
Worked solution
- 1
Compare like units.
The estimate and the scale reading are both in kilograms.
- 2
Find the gap.
Subtracting the smaller mass from the larger gives how far apart they are.
- 3
State which way the estimate missed.
The estimate is below the measured mass, so it was one kilogram too low.
The estimate was 1 kg too low.
Try it yourself.
Check your understanding
A bottle holds about 1 L. Is 950 mL or 9 mL closer?
Show answer & reasoning
950 mL
One litre equals mL. The choice mL is only mL below that benchmark; mL is much smaller, so mL is closer.
Keep practising
6 worksheet questions
Read the questions online
- Choose a sensible door height: 2 m or 2 cm.
- Choose a sensible teaspoon capacity: 5 mL or 5 L.
- A bag weighs 4 kg. Was an estimate of 3 kg too high or too low?
- A page measures 29 cm. Which estimate was closer: 30 cm or 20 cm?
- Name a tool to check an estimated table length of 1 m.
- Estimate the capacity of a small drinking cup. Explain your unit.
Read the worked solutions
- Choose a sensible door height: 2 m or 2 cm.
2 cm is a tiny length.
2 m - Choose a sensible teaspoon capacity: 5 mL or 5 L.
Litres suit much larger containers.
5 mL - A bag weighs 4 kg. Was an estimate of 3 kg too high or too low?
kg.
Too low by 1 kg. - A page measures 29 cm. Which estimate was closer: 30 cm or 20 cm?
30 is 1 cm away; 20 is 9 cm away.
30 cm - Name a tool to check an estimated table length of 1 m.
Use a tool that measures length.
A metre ruler or measuring tape. - Estimate the capacity of a small drinking cup. Explain your unit.
A cup holds part of a litre.
Other sensible cup-sized estimates are acceptable.
Example: about 250 mL.
Curriculum reference & lesson scope
Australian Curriculum v9 and SA Curriculum
Length/mass/capacity units; estimation; reading labelled measuring instruments.
Official curriculum source ↗