Three-quarters of Britons regularly assume they have more money in their current accounts than they actually do, and on average overestimate their bank balance by more than £70, says a report.
Research by Barclays shows 83% of people don't always keep track of how much cash goes in or out of their account. The average account holder checks their balance only four times a month.
Typically people think they are £70.73 better off then they really are, with Londoners overestimating their bank balance the most - by a margin of £91.62.
The study also found that people who used mobile banking are more aware of the state of their finances, with only 17% saying they did not always know what's going on in their account.
Sean Gilchrist, Barclays Digital Banking director, said: "Being in control of your money starts with knowing how much you've got and where it is being spent.
"Online and telephone banking made that easier and now mobile phone banking is taking it a step further."