When carrying out any written calculation it is well worth carrying out a quick mental estimation first and then using this estimation to check that the answer, once worked out, is sensible. It is surprising how often a silly answer goes unchecked because an estimation has not been carried out.
We have just published two sets of worksheets on estimating the answers to written addition in our Year 4 section. The first looks at rounding 3-digit numbers to the nearest hundred and the second looks at rounding four digit numbers to the nearest thousand. The two numbers are rounded to the nearest hundred or thousand and the rounded numbers added. For this exercise we do not ask for the children to work out the correct answer, although this could be done as well.
It is perhaps surprising that some children do not like the vagueness of an estimated answer and will try and mentally work out the exact answer in their heads. Whilst there is nothing wrong with this it is the almost instant estimation which takes just a second or two to do which is most helpful, especially when calculators are used in secondary school.