Estimates of the number of fighters in the ranks of the Islamic State in Iraq and the Levant (ISIL) are extraordinarily wide-ranging. On the low end of things, CNN’s Barbara Starr recently reported that “U.S. intelligence estimates that ISIL has a total force of somewhere between 9,000 to 18,000 fighters.” In late 2014, the CIA’s estimate of ISIL’s numbers was slightly higher, as its analysts assessed that the group had between 20,000 and 31,500 fighters between its Iraq and Syria holdings.

Other estimates are far higher. Rami Abdel Rahman, the director of the Syrian Observatory for Human Rights, has said that ISIL has more than 50,000 fighters in Syria alone. The chief of the Russian General Staff recently said that Russia estimates ISIL to have “70,000 gunmen of various nationalities.” In late August of 2014, Baghdad-based security expert Hisham al-Hashimi claimed that ISIL’s total membership could be close to 100,000. By November, Fuad Hussein, the chief of staff to Kurdish president Massoud Barzani, told Patrick Cockburn of The Independent that the CIA’s estimates were far too low, and that ISIL had at least 200,000 fighters.

Given this range of estimates, questions naturally arise: Who is right? Which estimate is closest to ISIL’s true numbers? To assess these questions, it’s necessary to consider which parts of ISIL’s force the estimates are attempting to count, the total amount of territory ISIL is occupying, and the attrition that coalition forces have inflicted upon ISIL. Bearing in mind all of these factors, it becomes clear not only that the high-end figures are plausible, but also that they are far more likely than the unrealistically low numbers propounded by U.S. intelligence.

The figure of 200,000 ISIL fighters advanced by Fuad Hussein includes support personnel (ansar), police-style security forces (hisba), local militias, border guards, paramilitary personnel associated with the group’s various security bodies (mukhabarat, assas, amniyat, and amn al-khas), and conscripts and trainees. The actual number of ISIL front-line and garrison fighters is much lower, which are divided between their regular forces (jund), the elite paramilitary (inghimasiyun, which alone may have up to 15,000 members), and death squad (dhabbihah) personnel. Unless one is able to objectively evaluate these bodies, merely throwing out raw numbers is meaningless.

Turning to ISIL’s holdings, it’s worth reviewing the territories that ISIL is believed to occupy in Syria, along with their population sizes:

Raqqa province (population 944,000)

Dayr al-Zawr province, not including Dayr al-Zawr City (746,566)

Shaddadi, Markada, and al-Arish districts of Hasaka (90,095)

Jarabulus and Manbij districts of Aleppo (467,032)

This totals a population of 2,247,693 in Syria alone for ISIL to administer, while both imposing extreme Islamic rule and sustaining large-scale offensive operations elsewhere. For comparison, in Afghanistan it took Regional Command Southwest around 30,000 U.S., U.K. and Danish troops to attempt to subdue Helmand and Nimruz, which have a combined population of 1,598,369. Further, these coalition forces had the help of the Afghan National Army, the Afghan National Police, the Afghan Community Order Police, and the National Directorate of Security, meaning that the total forces on their side were far more than 30,000. Even accounting for terrain and infrastructural differences, and the fact that ISIL does not possess the long tail of modern Western forces (i.e., the logistics necessary to sustain a modern fighting force, technological specialists, and a vast intelligence infrastructure), ISIL would still need, at minimum, a comparable force of around 30,000 just to maintain their Syria-based holdings.

The fact that U.S. intelligence estimates of ISIL’s force size are too low becomes evident when one considers the group’s current Iraq holdings, along with their population size:

Mosul, Hamdaniya, Tal Afar, al-Hadar and Ba’aj districts of Ninawa (population 1,984,829, reduced to around 1,484,829 due to the reported flight of 500,000)

Al-Dibs, Daquq, and Hawija districts of Kirkuk (525,758)

Al-Qa’im, Rutba, Anah, Hit, and Falluja districts of Anbar (1,767,686, likely reduced to 1,587,686 due to the reported flight of 180,000 from Hit)

Al-Sharqat, Tikrit, and Dawr districts of Salahaddin (367,244)

In Iraq, that’s a population of 3,965,517 to 4,645,517 that ISIL has to control—about twice the size of the group’s Syria holdings. And these territorial holdings come on top of multiple ISIL contingents that maintain their own logistics and support personnel, and that are capable of carrying out battalion-sized offensive operations, including the al-Sarim al-Battar, al-Aqsa, Grozny, Sarajevo, Yarmuk, Jalut, Dawud, Jabal, Saiqa, Zilzal, al-Qa’qa, Hitin, and al-Qadisiyah battalions. (Note that “battalion” does not mean the same thing for ISIL as it does for U.S. forces: ISIL’s battalions are not as large.) With these factors in mind, Hisham al-Hashimi’s estimate of ISIL having 100,000 men under arms appears plausible. Further, if one takes into account ISIL’s establishment of multiple security bodies and the mass conscription it has imposed in Raqqa, Ninawa, and western Anbar, the overall Kurdish estimate of 200,000 men may be plausible as well.

In addition to ISIL’s force structure and the extent of its holdings, another indication that U.S. intelligence estimates are too low is the amount of attrition that has been inflicted upon ISIL. CENTCOM has said that it has killed 6,000 ISIL fighters in airstrikes alone since August. If the low-end estimates are accurate, then between 20-30% of ISIL’s total manpower has been eliminated by airstrikes. Such a conclusion is clearly unrealistic: Even if ISIL were able to replenish its ranks at a rate equal to the attrition, the group would be performing far worse on the battlefield if it had had to replace such a large percentage of its force in such a short period. By contrast, if you accept Hashimi’s figure of 100,000 ISIL fighters, the group has only lost about one-tenth of its total force.

It still isn’t clear precisely how many fighters ISIL has, but its total force is likely to be closer to 100,000 than to 30,000 (although, unlike the martyrdom-seeking fanatics in its ranks, ISIL’s conscripts are more likely to turn tail and run in a tough situation). The low-end estimates are simply too low to be realistic, while the high-end estimates—of which many observers are intuitively skeptical—are far more plausible than they first appear once one attempts to break them down more systematically.

Daveed Gartenstein-Ross is a senior fellow at the Foundation for Defense of Democracies (FDD) and an adjunct assistant professor in Georgetown University’s security studies program. FDD’s Oren Adaki contributed to the Arabic-language research for this article.