The official U.S. unemployment rate is 5.1% -- the lowest in seven years -- but Donald Trump calls that a "joke."

On Monday, he claimed he'd seen numbers that show America's real unemployment rate is as high as 42%.

How does he arrive at such a wild figure?

Trump appears to be looking at the number of American adults not working. Period.

Out of about 251 million American adults, roughly 102 million -- or 40.6% -- aren't working.

But, of course, there are a lot of reasons people don't work. They could be disabled, in college, at home raising kids or retired.

And you wouldn't include those groups in the unemployment rate, which tries to capture how many people are actively looking for work and can't find it.

Trump's claims getting more outlandish

The 42% claim just marks a ratcheting up of his rhetoric to show that America needs a drastic fix.

When he announced his run for president, he said "the real unemployment is anywhere from 18% to 20%."

CNNMoney (among others) already called out that claim for being false. The broadest measure of unemployment is 10.3%, a rate officially called "U-6 unemployment," which includes those who work part-time but want a full-time job, as well as people who have given up looking for work in the past year.

There's a valid, but different, conversation to have about the trend that more and more American adults aren't working. It was expected that the number of American adults in the workforce would decline as more Baby Boomers reached retirement age.

Economists like to point out that the two main drivers of growth are more people working or else higher productivity. With fewer adult Americans working, that makes it more difficult for the country to grow.

A common solution to that problem is letting in more immigrants, but it's hard to imagine that under a Trump administration. He even wants to deport 11 million undocumented immigrants.

"He will be lucky to get negative 3% growth if he got his wish," says Mark Zandi, chief economist at Moody's Analytics.