Estimating using rounding

We can use rounding numbers to get a rough idea or an estimate. An estimate might be a little more or a little less than the actual amount.
By carrying out an estimate we can check that the answers to problems are sensible.

If you were buying 9 identical shirts for the school's sports team that cost £7.80 each, to get a rough idea of the total cost you could round up £7.80 to £8.00. You could also round up 9 shirts to 10 shirts.

Your calculation would then be
10 x £8.00 = £80.00

The actual cost would be
9 x £7.80 = £70.20

Notice that the actual cost of £70.20 is a little less than our £80.00 estimate. This is because we rounded up.
When using a calculator it is a good idea to estimate the answer first in case you make keying errors.

Example

To estimate the cost of 11 pens at 95p each, you could round down 11 to 10 pens and round up 95p to £1.00
The estimated cost would then be
10 x £1.00 = £10.00