The government shutdown that began on Saturday is likely to slow the economy, inconvenience millions of Americans and leave both President Donald Trump and Congress with a political black eye. But at least shuttering the government for a while will save taxpayers some money, right?

Actually, no.

Shutdowns end up being more expensive than just keeping the government open—and this one, if it’s like past shutdowns, will likely cost tens of millions of dollars a day. Maybe more. And it's a mystery even to experts just how much.

A shutdown seems at first like it would lead to some quick and dramatic savings. Non-essential employees are furloughed, the government stops writing checks for huge swaths of programs and even essential employees come to work without getting paid.

But in reality it doesn't quite work like that. Budget experts and past analyses by the White House budget office have found that a shutdown hurts the U.S.’ finances in a number of ways. Furloughed workers almost always get paid retroactively for the time they were out—which means taxpayers are laying out money without getting any work in return. Museums and national parks can’t collect fees and revenues from other sources like gift shops. Perhaps most importantly, federal workers spend thousands of cumulative work hours preparing for the event and recovering from it, literally shutting down their systems and then restarting them once the government reopens—paid work that is utterly unnecessary to the normal business of running the country, and sucks time away from safety inspections, or reviewing research grants, or whatever their actual responsibilities are.

Quantifying the exact cost to the government is difficult, in part because every shutdown is different. Between November 1995 and January 1996, the government shut down twice for a total of 27 days as Democrats and Republicans clashed over Medicare funding, among other issues. A subsequent analysis conducted by the White House’s Office of Management and Budget estimated that both shutdowns together cost the government $1.4 billion—more than $2 billion today after adjusting for inflation. “That's not monopoly money,” then-President Bill Clinton said in January 1996 as the two parties were on the verge of yet another shutdown. “Shutting down the government again would be unbelievably irresponsible.”

Of that $1.4 billion, roughly $1.1 billion was salary paid to federal workers who stayed home and didn't work. The remaining $300 million came from other sources, such as the lost revenue from the closure of national parks and public museums. According to the budget office, the shutdown resulted in the cancellation of seven million visits to the national parks and two million visits to museums and other cultural sites, such as the Smithsonian and Kennedy Center.

There are also more abstract costs—real, but hard to measure. Lower economic growth means less money coming into the national treasury as federal workers have less money to spend and companies postpone investments, either because key economic data is delayed or simply out of uncertainty about major upcoming policy decisions. Small businesses don’t receive loans, research grants are delayed and federal contracts are postponed—all actions that suck money out of the economy. And the lost productivity from workers doing shutdown-prep work instead of their jobs hasn't been calculated, but could easily total in the billions of dollars, experts believe.

The costs for a government shutdown appear to be growing larger as well—or the government is becoming better at estimating them. In a report on the consequences of the 16-day government shutdown in October 2013, the Obama administration estimated that the payroll cost alone due to retroactive pay was $2 billion, rising to $2.5 billion if you include all forms of compensation, such as workplace benefits. That was a result of 6.6 million combined furlough days among federal workers, more than any previous shutdown. There were other costs as well: The Internal Revenue Service, the report found, couldn't pursue its normal enforcement activities during the shutdown, which typically bring in roughly $1 billion a week. More popular government agencies took smaller but meaningful hits: The National Park Service missed $7 million in fees; the Smithsonian lost $4 million.

Economic forecasters also estimated that the 2013 shutdown shaved $2 billion to $6 billion off fourth-quarter GDP growth that year as furloughed employees have less money to spend on restaurants, shops and other establishments. Some of those losses were likely made up during 2014 as federal workers received their retroactive pay and the economy rebounded. But the shutdown undoubtedly caused some lasting harm to the economy, which ultimately could cost taxpayers millions of dollars in reduced income tax payments.

Some budgetary costs are more difficult to estimate: Under the Prompt Payment Act and Cash Management Improvement Act, the government must pay its bills within 30 days—or else pay interest on any late payments. It’s unclear whether any federal agencies missed the 30-day deadline due to the shutdown in 2013. But if this one goes on long enough, any resulting interest payments impose yet another unnecessary cost on taxpayers.

One reason it's hard to gauge the costs exactly is that shutdowns are more subjective than they might seem: Agencies change their shutdown plans frequently, and different administrations interpret the law differently. For instance, while the Interior Department shuttered national parks during the 2013 shutdown, Interior Secretary Ryan Zinke said the agency intends to keep them open this time around, potentially allowing them to continue collecting fees despite the shutdown. It’s also unclear how many workers will be furloughed during this shutdown, or whether they will receive retroactive pay, which Congress typically authorizes by passing a law—but there’s no guarantee that it will do so this time. Not paying them would keep some money in the government’s pocket, but at the expense of the people and families who would normally have used and spent the money—a loss either way, from an economic point of view.

To be sure, the costs of previous shutdowns—a few billion dollars here and there—are a drop in the bucket relative to total federal spending, which is roughly $3.5 trillion each year. But the longer the shutdown lasts, the costlier it becomes—and eventually it could start taking a big bite out of the treasury. The greatest risk is that investors become nervous about the U.S.’ willingness to pay its debt on time, and demand higher interest payments on Treasury bonds. Small changes in interest rates can costs the government huge amounts of money: The Congressional Budget Office estimates that a 1 percentage point rise in interest rates—for whatever reason—would increase the deficit by $1.6 trillion over a decade.

A short government shutdown is unlikely to affect the interest rates on Treasury bonds much. But the longer the shutdown, the more investors will fear that the legislative branch of the government has broken down and can't collaborate to make crucial fiscal decisions; they will become especially nervous as the government nears the debt ceiling, at which point it can no longer borrow and must pay its bills with any incoming revenues. Since the U.S. spends more each year than it takes in, it would have to pick and choose what bills to pay.

Treasury most recently projected that the government will hit the debt ceiling near the end of February, although that date could change given the shutdown. The U.S. has never breached the debt ceiling before for a sustained period of time, and the exact consequences of doing so remain hotly debated among economists. But as the shutdown stretches from days to weeks, that risk grows exponentially—and so could the costs for the American taxpayers who ultimately foot the bill.

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