by Allen Wyatt
(last updated August 17, 2019)
Lynn uses Excel to keep track of her checkbook. She has the cells in the worksheet formatted to show values like $1,234.56. Sometimes, but not always, if Lynn changes the formatting of a cell it may look like 1234.56000000001. This seems like an error to her and she finds it annoying, so she is wondering why the figures look this way at times.
Lynn has inadvertently discovered the strange, wonderful world of Excel math. You see, when you do math in an Excel formula, because of the way that Excel does the calculations internally, you could easily end up with a rounding error at the most extreme ends of the calculation. (Excel maintains internal precision to 15 significant digits, which is why Lynn is seeing the "error" out at the very furthest reaches of her number.)
In routine calculations—like the ones being done by Lynn—this typically isn't a problem. In her usage, she only cares about numbers being correct to a precision of two decimal places. When the "error" is in the eleventh decimal place, it isn't going to affect the validity of what Lynn sees. It would only affect the results if a particular result relied upon thousands and thousands and thousands of previous calculations. This is the only way that the "error" could compound to the point where it affects Lynn's results.
In day to day usage, this shouldn't be an issue. The "error" is hidden easily by using a cell format such as Currency or Accounting that displays values, so they show only two decimal places.
If the rounding issues bug you a lot, the easiest solution is to force your formulas to round to the number of decimal places you want. For instance, let's say you have a very simple formula such as the following:
You could "wrap" the formula in the ROUND function, in this manner:
This forces the result returned by Excel to two decimal places, which is what Lynn would expect to see if she removes the cell formatting.
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