Magento discount calcuation 1p off

Question

I have setup a shopping cart price rule of 10%. If I add a product value 33.95 in the cart and apply that rule, it shows discount of 3.40 which is suppose to be 3.39. I am assuming its rounding up to 2 digit and may be needs changing to 3 digit (I'm assuming this) but don't know where to make this change.

Answer 1

Welcome to the world of rounding errors in Magento.

Usually, it is not a cause for major concern, as the cart total is correct; but the row level items are calculated wrong. This also effects the admin order display and invoice PDFs.

Different versions have different rounding bugs, there isn't a fix in the strictest sense, we just patch them as we go along.