This article has been corrected.

It’s hard to overstate the extent to which Donald Trump’s campaign-trail mentions of building a wall on the border with Mexico was more applause line than policy proposal. Beyond the ludicrous argument that Mexico would pay for its construction — an argument that has been revealed as empty so often that it is now its own punchline — Trump also regularly used the height of the wall as an exclamation point. Any time the president of Mexico said something Trump didn’t like (like that his country wouldn’t pay for the wall) the wall “got 10 feet higher,” as Trump said several times during the campaign.

For a while, we tried to track the actual height of the wall, assuming that Trump actually planned to add 10 feet every time he said he was going to.

The wall started here two months after he declared his candidacy in 2015.

(Trump added for scale.)

By the time we stopped updating our tracking of the wall’s height, it was here:

Last week, the White House sent Congress its first formal estimate of the cost of the first phase of wall construction, including a sense of scale. The wall would range from 18 to 30 feet in height, and the first phase of construction would include 316 miles of new construction and reinforcement of 407 miles of existing fencing.

So, the wall would end up somewhere between these two heights.

The price tag for this effort? A paltry $18 billion over 10 years. Just a bit more than the annual cost of the federal judiciary (which cost $17.4 billion in 2017).

How unrealistic is it that Mexico would foot that bill? The $18 billion cost would account for about 1.7 cents of every dollar the country generated in gross domestic product in 2016.

Naturally, a 316-mile-long, 30-foot-tall wall is expensive. But we were curious about something. Eighteen billion is a lot of dollars. If you were to create a wall out of actual dollar bills, how far would it extend?

As it turns out, an 18-foot-tall wall can be built out of $18 billion in dollar bills and stretch … 31.8 miles, one-tenth of that distance.

The dimensions of a dollar bill are about 156 millimeters in width, 66.3 millimeters in depth and 0.1 millimeter in height. (We used millimeters since Mexico is paying for it.) A stack of 54,860 bills reaches a height of 18 feet; a stack of 91,440 bills gets you to 30 feet. (We ignored the effects of compression here, meaning the fact that you could fit more bills in a stack given that the weight of the bills at the top would compress the bills at the bottom. We do need to inject some sort of adhesive between the bills, so let’s call it a wash.)

We end up with a paper column that’s 18-to-30-feet tall and about 6 inches wide. So how far would that stretch, if you lined up these columns one next to another?

Divide a 18 billion by 54,860 or 91,440 and multiply by 156 millimeters inches. An 18-foot-wall, as above, gets you 31.8 miles. The taller wall goes only 19 miles.

Of course, you also end up with a wall made out of paper, which is not terribly robust as a deterrent. (Not to mention a wall made up of negotiable currency.) What if we wanted a metal wall?

Happily, the United States also makes a metal currency. So we looked at how big a wall could be made with $18 billion in pennies.

Pennies are narrower than dollar bills, as you may be aware, measuring three-quarters of an inch in diameter. They are also a bit thicker than a dollar bill, at 1.52 millimeters.

For a column 18 feet in height, you need 3,609 pennies. For a 30-foot wall, you need a column 6,016 pennies high.

How far would that stretch? The trick here is that a penny has — ahem — 1/100th the value of a dollar bill. So while pennies are narrower than bills, you need a lot more of them to make up the same value.

In short, you could build a 5,904-mile wall out of pennies that’s 18 feet high, or a 30-foot wall that runs for 3,542 miles. Given that we’re talking about a wall that is only three-quarters of an inch thick, we could build it 18 feet high and three pennies thick (2.25 inches), which still allows us to run 1,968 miles — nearly the length of the border. Or we could run it the length of the borders with Mexico and Canada, which, excluding Alaska, is a little over 5,900 miles.

(A penny is actually worth more than a penny, by the way, thanks to the copper and zinc that compose them. $18 billion worth of pennies, then, would actually be fewer than 1.8 trillion pennies. But we’re not talking about actual value here, since the value of the paper in a dollar bill is much less than a dollar.)

How much copper would be used in this wall? A new penny weighs 2.5 grams, 95 percent of which is zinc and 5 percent of which is the copper exterior. In total, then, the 1.8 trillion pennies worth $18 billion would use 4.7 million tons of zinc and 248,000 tons of copper. At the current trading price of copper, that’s about $1.6 billion in copper costs alone.

There are other costs that would be required for this wall, too. Adhesive, as mentioned above. Some sort of support structure: An 18-foot-tall tower of pennies would likely be prone to toppling over, which wouldn’t be useful for a wall. Trump has also argued that the wall needs windows in it, so that people walking near it on the American side wouldn’t be hit and killed by bags of drugs being thrown over the wall by drug dealers.

This was not incorporated into our analysis.

Correction: The original version of this article misplaced a decimal point, lamentably. The figures have been corrected as a result.