“Very wealthy people often have large investments they’ve held for long periods, and if those investments aren’t sold or traded, that money is never taxed,” Gates wrote in a blog post. "That doesn’t make sense.”

Capital gains taxes should go up too, “probably to the same level as" ordinary income, he said. The estate tax should be hiked, and loopholes used to duck it ought to be shut down. People should also pay more on “carried interest,” Gates said.

He also called for higher state taxes, including the creation of an income tax in his home state of Washington.

His comments come as Democrats have rallied around higher taxes on the rich.

Gates rejected the notion that the wealthy could simply volunteer to pay more.

“Leaving it up to people to give more than the government asks for is not a scalable solution,” he wrote. “Additional voluntary giving will never raise enough money for everything the government needs to do.”