Type this into a browser console (F12 → Console):

0.1 + 0.2          // 0.30000000000000004
0.1 + 0.2 === 0.3  // false

Python prints the same. It looks like a bug, but it is how almost every computer stores decimals.

The reason: binary fractions

In decimal, 1/3 cannot be written exactly — it is 0.3333… forever. In binary (base 2), the same thing happens to 0.1: it becomes an endless repeating pattern. Computers store numbers in a fixed number of bits (the IEEE 754 “double” format, about 15–17 significant digits), so 0.1 is stored as the nearest possible value, very slightly off. Add two slightly-off numbers and the tiny error becomes visible.

What Excel does

Excel uses the same double format but displays at most 15 significant digits, so =0.1+0.2 shows 0.3. The error is still there:

=0.1+0.2-0.3          →  5.55112E-17  (not 0)
=1-0.9-0.1            →  -2.77556E-17

That is why a reconciliation that “should be zero” sometimes shows -0.00, or a lookup on a calculated value fails.

How to avoid the bugs

• Round money at the right step: =ROUND(A2*B2, 2) before comparing or summing.
• Compare with a tolerance: =ABS(A2-B2) < 0.005 instead of =A2=B2.
• In code, use decimal types for money: Python decimal.Decimal("0.1"), or store paise/cents as whole numbers.
• Do not use “Set precision as displayed” in Excel options unless you understand it — it permanently changes stored values.

More surprising computer facts in the Wacky Facts collection.