Republican presidential candidate Ben Carson said on Sunday his proposed flat-tax plan of about 10 per cent was inspired by the biblical practice of tithing.

"You make $10 billion a year, you pay a billion; you make $10 year, you pay one. That's pretty damn fair if you ask me," Carson said on "Fox News Sunday."

Carson, a former neurosurgeon who announced his presidential campaign last week, has cast himself as a non-politician and a problem-solver in an attempt to set himself apart from the Republican field.

He is a long shot in the race, but his idea of a single, proportional tax may resonate with conservative evangelical Christians who believe in tithes and with Americans frustrated by a complicated tax system.

"I like the idea of a proportional tax - that way you pay according to your ability," Carson said. "I got that idea quite frankly from the Bible."

Carson disputed the idea that his plan could hurt the poor, saying that it is condescending to assume they would not be able to pay.

"Poor people have pride, too, and they don't want to be just taken care of," he said, adding that his plan would eliminate loopholes in the US tax system and make it hard for politicians to raise taxes.