# Excel SUM anomaly

Excel can’t add up reliably and it never has. It’s an important limitation of computer software that everyone needs to be wary of.

Phil N sent us an interesting Excel worksheet.  He’s simply adding a column of numbers starting with a negative number then positive numbers – the total should be ‘a wash’ – that is add up to zero.

-127551.73
103130.41
1807.75
7390.11
9028.59
2831.26
1568.90
1794.71

According to Excel initially the total is zero but that cell display is misleading.

When you expand the number of digits to the right of the decimal point you see that Excel has come up with a screwy result.

With General cell formatting, that’s really obvious.

If you’re just looking at a few decimal places the problem won’t be apparent.

You might think that these sorts of issues don’t matter but it becomes a problem when you do tests on the result – for example a simple IF test to see if the result is zero will return a False answer.

There’s also an argument that the responsibility is on users to put proper tests into their worksheets to avoid these issues – that’s true to some extent but the primary responsibility is with Microsoft.  Excel should be able to handle simple addition as well as my handheld calculator or an elemetary schoolchild.

The surprise in Phil’s example is that it involves simple addition – often there’s hassles with multipy or divide which have long remainders.  But it’s hard to understand why the addition of numbers to 2 decimal places gives a result with a remainder in the 9th decimal place and beyond.

Here’s the worksheet in Excel with a IF statement test:

The test formula is simple: `=IF(A9=0,"Yes","No")`

It’s revealing to look at progressive SUM results starting with adding all cells A1:A8 as above then removing one cell each time down to A1:A2.

As you can see, the problem starts when adding A1:A5 and beyond.