More than half (53 percent) of mobile broadband don't know how much data they use a month, says Broadband Genie.
Research by the broadband website, revealed that of those that are aware of their mobile data usage, 11 percent use up to 1GB, while 13 percent use up to 5GB.
Just under one in ten (nine percent) said they use between 5 and 10GB of data per month, while 14 percent get through 10GB or more of data per month.
"Consumers misunderstanding data limits is unfortunately common. Many customers are paying too much for their mobile broadband data tariff. We're talking about thousands of customers who could probably halve their mobile internet bill and see no difference in their online usage," said Broadband Genie editor Chris Marling.
Marling said that with a massive range of deals available, currently from £35 per month for 20GB of data to as little as £10 per month for just 1GB, consumers need to be aware of their usage, or literally pay for the consequences.
"Many people wont think to check their bill unless it's higher than normal – if it's just the amount they expect, they don't think to see how close they are to their limit," he said.
"We strongly advise people to go back over their bills to see how much they could potentially save, either by reducing their tariff or even going to a pay-as-you-go deal."
See also: Mobile broadband hit 271 million in 2009