Counting ISIS fighters is hard.
For starters, there's one of the first U.S. estimates of ISIS fighters from September 2014.
"We're not just simply about degrading and destroying them, the individuals, the 20,000 to 30,000," said Press Secretary Rear Admiral John Kirby during a Pentagon press briefing. "It's about degrading and destroying their capabilitites."
That's different from the U.S. estimate of ISIS fighters killed 23 months later.
"When you add that to the 20,000 estimated killed prior to our arrival, that's 45,000 enemy taken off the battlefield," Lt. Gen. Sean MacFarland said during a Pentagon press briefing.
So, does that put ISIS at -15,000 fighters? Not quite. Estimates on just how many fighters ISIS has vary — by a lot.
There are estimates as high as 200,000, such as the one made in 2014 by a senior Kurdish leader. There are also estimates as low as 9,000, which was the Pentagon's lowball number early on.
MacFarland acknowledged the challenge of making an accurate estimate, telling reporters, "The thing about these numbers is that they're pretty soft. Pretty squishy. That's why we don't typically quote them a lot."
ISIS still controls Mosul in Iraq and Raqqa in Syria — both rather large cities. In 2015, Mosul's population was estimated to be anywhere from 1,000,000 to 1,694,000. Raqqa's is thought to be somewhere between 250,000 and 500,000.
Both cities, along with the populations throughout the other territory it controls, give ISIS the potential to swell its ranks with forced recruitment.
