Trying to take an inventory of ISIS is a daunting task, even for those senior analysts in the intelligence community — few rely on the annual statistical reports that the media-savvy group actually releases.

But experts have been able to provide some estimates on how big, how strong and how rich the militant organization may be.

"When you hear the intelligence community say they don't even know, that's somewhat worrisome," said Colin Clarke, an associate political scientist at the Rand Corporation, which specializes in insurgency and transnational terrorism.

10,000 to 20,000: The number of fighters

Clarke pegged the ISIS fighting force at around 10,000, a far cry from some estimates that have placed that number as high as 80,000.

"When people throw out these numbers, I'd ask them what do they define as a fighter," said Lauren Squires, a counter-terrorism analyst at the Institute for the Study of War.

“We look at the battle-hardened battalion manoeuvre​ groups. We think of that as up to 20,000 [fighters]."

Austin Long, assistant professor of international and public affairs at Columbia University and a former adviser to the multinational force in Iraq, said the number of "true believers" probably numbers in the low thousands. But there are also those who are part of the fighting because they see ISIS as "the winning team," which he said could put the force at around 15,000, although he says 20,000 is also possible.

$1M to $4M: The amount of revenue ISIS takes in a day

Generating revenue is one of the organization's biggest strengths and it has historically, been “fantastic” at generating revenue from a lot of different sources, Long said.

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Unlike al-Qaeda, which receives much of its funding from wealthy Gulf state patrons, ISIS self-generates much of its revenue, said Clarke. Carjackings, bank robberies, extortion, kidnappings for ransom are all used to fund its organization.

"The group is like the Mafia. It really doesn’t discriminate in how it gets its money," Clarke said. "Anything that's a revenue generating activity, this group is engaged in."

How much it has and how much it takes in is again difficult to discern. Some estimates believe ISIS takes in around $1 million a day, while others estimate around $25 million to $30 million a year.

Squires said their institute pegged it higher, at around $2 million to $4 million daily.

Regardless of the number, Clarke said ISIS’s financial resources go “well beyond” anything al-Qaeda had at its apex with Osama bin Laden at the helm.

Acquiring territory, however, also comes with the expense of running the areas it controls.

"This isn't money that's just sitting in a piggy bank," Clarke said. "As the group grows its recruiting base, it has to pay these fighters. It has to pay for this. It's establishing a caliphate and running a caliphate is not cheap. There's money going out, not just coming in."

3 divisions worth: ISIS's stock of weapons and equipment

"When you look at them as a conventional force, they have approximately at least three divisions — if you think of a Western military division worth of equipment," Squires said.

ISIS generates much of its revenue through carjackings, bank robberies, extortion, and kidnappings for ransom. (Stringer/Reuters)

"But these are only operable if ISIS is undergoing a conventional operation. They are more lethal and more productive as a terrorist organization."

It’s really less about quantity than it is about the quality of weapons, Clarke said. ISIS has raided a number of Iraqi military depots, acquiring all sorts of weapons, including sniper rifles, mortars, heavy machine guns, anti-tank weapons, RPGs, tanks, and Humvees.

Most troubling, says Clarke, is ISIS’s recent grab of portable air defence systems called MANPADs — shoulder-mounted homing missiles that are relatively simple to use and can reach aircraft flying as high as 5,000 metres.

90,000: The number of square kilometres ISIS is believed to control

The amount of territory held by ISIS changes by the day, said Clarke but it includes big swaths of north and northeast Syria to include major cities like Aleppo and Raqqa. ISIS also has a major presence around a critical border crossing between Syria and Iraq. In Iraq, ISIS's reach stretches to the northern and northwestern parts of country, but also goes as far down as Fallujah and west of Kirkuk.

The New Yorker estimated that, in total, ISIS controls about 90,000 square kilometres of land — about the size of Jordan.