The consumer group made its findings after asking 18 volunteers, including a principal inspector of taxes and a retired headteacher to calculate from a mock statement what this cost would be by looking at banks' and building societies' charging structures on their websites.
The volunteers got just 10 out of 72 calculations correct between them, with the tax inspector getting just one of his four calculations right and the former headteacher getting them all wrong.
It also took people 10 minutes on average even to find the charges on websites - and in some cases it took longer than half an hour.
The research found that variations in language used to describe unauthorised overdraft charges caused further confusion. The terms were found to include "informal", "unplanned", "unarranged" and "unapproved".
Some banks have started to simplify their charges, but Which? argued this does not necessarily make it easier for people to find the best account for their needs.
Which? wants the Government to force banks to release the data they have about how customers use their accounts, which could be used to allow consumers to rank providers by cost.
Figures released by the Payments Council showed there had been a recent upswing in people swapping bank accounts following the launch of a new industry guarantee to help shake up competition.