Bill Gates' fortune is so vast that the US government hasn't got a computer big enough to store the financial history of his wealth.

The Microsoft founder has admitted that the US tax office has to store his financial data on a special computer because his fortune is so vast.

"My tax return in the United States has to be kept on a special computer because its normal computers can't deal with the numbers," he said at a Microsoft conference held in Lisbon.

"So I am constantly getting these notices telling me I haven't paid something when really it is just on the wrong computer.

"Then they will send me another notice telling me how bad they feel that they sent me a notice that was a mistake."

According to the latest Forbes magazine, Gates's has an estimated fortune of US$47 billion (A$62.88 billion).

Gates and his wife Melinda have created the world's biggest charity with an endowment worth US$29 billion (A$38.8 billio