Donald Trump has made a lot of big promises. Among the most ambitious is his vow to “create 25 million new American jobs in the next decade and return to 4% annual growth.”

That’s a lot of jobs to create. Even trickier than creating those jobs, however, will be finding American workers to fill them. Trump’s stance on immigration makes it unlikely that the US will be importing many foreign workers. So where will they come from?

Let’s walk through the math. Currently, the US employs about 152 million workers (pdf, p.7). Trump’s proposal would therefore up the size of America’s workforce to 177 million, an increase of around 17%. The Congressional Budget Office expects population growth to boost the number of US workers by about 8 million by 2027, argues Ben Zipperer, an economist at the Economic Policy Institute, a left-leaning research nonprofit group.

That leaves 17 million more workers to drum up.

Of course, the US has prime-age workers waiting in the wings right now. But as Free Exchange calculates, even restoring the participation rate of all workers under the age of 64 to the historic high in 1998 would swell America’s workforce by only around 8 million people.

That still adds up to around 9 million of Trump’s new American jobs that will need filling. The math gets even hairier if you take Trump at his word on deportations of immigrants who are in the US illegally. Though it’s hard to estimate with certainty, at least 7 million people working in the US today are unauthorized immigrants. If the president makes good on his plans to deport even just a fraction of them, that’s even more want-ads to post.

The only other option for Trump to make good on his promise is to hire elderly workers. The share of those aged 65 and older who would have to join the workforce would soar to 32%, up from the current 19%, according to EPI’s Zipperer.

“Having the elderly work more is problematic for two reasons. First and foremost are our social priorities: shouldn’t a growing rich country make it easier, not harder, for its older citizens to retire?” he says. “Second, older individuals are already working more in record numbers.”

This is bad news for Trump’s ambitious 4% growth plan, too. There are only two ways to grow an economy, in real terms: boost productivity, or simply add more workers. If Trump pulls off his planned infrastructure investment stimulus, that might help buoy productivity growth a little, but probably not enough to offset tanking manufacturing productivity.

That means the fate of Trumponomics will depend heavily on expanding the workforce. With expanded immigration off the table, getting America’s elderly to work longer is looking more and more essential.