A calculator gives an exact response to the keys entered, but it cannot decide whether the right numbers or operation were used. Estimating first gives the young person an expected range and makes entry errors easier to spot.

Estimate before pressing equals

Ask for a quick approximation, not a second full calculation.

For example:

• three items at £4.85 each should cost about £15;

• 19% of £80 should be a little less than £16;

• a 3.2 km journey repeated four times should be around 13 km.

Write the estimate beside the work if holding it in mind is difficult.

If possible, use the same kind of calculator as in lessons. A phone calculator may handle percentages and brackets differently from a school scientific calculator, which can give a surprising answer.

Compare the result

After using the calculator, ask:

• Is the answer close to the estimate?

• Is it larger or smaller in the way we expected?

• Does the decimal point look sensible?

• Are the units correct?

If the result is far outside the expected range, check the entered values and operation before repeating the whole question.

Avoid turning it into a test

Agree the routine with the young person and use it for selected homework questions. The aim is an independent checking habit, not an adult challenge after every calculation. Step back once the young person begins estimating and checking without prompting.

If estimates themselves are consistently unrealistic, ask the maths teacher which number line, familiar amounts or rounding method is being used in class.