Rough calculations

The idea is simple: round all the numbers in the calculation to something short enough for you to work with, and then do the calculation. The anwer wont be that close but it will probably be enough to spot calculator errors, for example.

Example 6224 x 1.93

You could round the first number to 6000, and the second to 2. The answer should be roughly 12,000 (actually it is 12012.32 ).

See also Improving rough calculations with percentages.