The National Counterterrorism Center estimates the territory occupied by the Islamic State at 81,000 square miles. However, the size is a matter of dispute among scholars for two reasons. First, the definitions of "occupied territories" differ: Some refer to areas in which fighting is still taking place, while others take into account only territory exclusively ruled by the Islamic State.
Active-duty fighters/ militants in combat
Most estimates of the number of fighters on active duty range around 20,000, while the CIA estimates a maximum of 31,500 fighters. Considering this, the core of the Islamic State would be comparable to the 21,600-strong military of Madagascar, which defends the world's fourth-biggest island. The number estimated for the Islamic State varies depending on whether you count only those who fight for the ideals and aims of the Islamic State itself — which is a smaller group.
"If you counted in those who have joined the Islamic State's conquest for their own benefit, the number would be much higher," says Austin Long, who teaches security policy at Columbia University. In the latter group would be those Sunni Arabs who have joined the Islamic State's fight to bring down the Shiite-led government in Baghdad — but don't share the goal of establishing a caliphate in the region.
Although politicians worry about the presence of foreign fighters in the ranks of the Islamic State, local fighters still account for the majority. A variety of studies estimate that foreigners (by which we refer only to people born in nations other than Syria, Iraq and their direct neighboring countries) account for 20 percent to 30 percent of the organization's manpower. There is no state with a percentage of foreign-born soldiers comparable to the Islamic State. France's Foreign Legion — which consists solely of foreigners and is distinct from its domestic army — makes up only 2 percent of the country's active-duty military personnel.
Oil production in barrels per day
Several oil fields are located within the territory occupied by the Islamic State. The Wall Street Journal estimates that the oil production of these fields could range from 30,000 to 70,000 barrels per day. The numbers vary because the Islamic State is thought to produce less oil than it could theoretically extract. An average production of 50,000 barrels per day would match the size of Bahrain's production, which is the lowest in the Persian Gulf region. This is comparable to the oil production of Mississippi (66,000 barrels per day).
Based on its oil revenue, ransom payments for kidnappings of foreigners and locals, and robberies of banks and other institutions within its occupied territory, the Islamic State's revenue is thought to exceed $1 million a day. This would match the Vatican's revenue — however, the Islamic State's actual figures could be much higher.
Favorable mentions on Twitter
Staffan Truvé, a researcher for the Internet technology company Recorded Future, found that the Islamic State was mentioned favorably on Twitter 27,000 times from Aug. 18 to Sept. 3. "After identifying all Twitter accounts that were talking about the Islamic State, we then compute what we call a set of sentiment metrics which looks at the words in a text to decide if it is positive, negative or violent, for example," Truvé told The Washington Post. Broadly, 11 percent of those who tweet about the Islamic State seem to have a favorable opinion, compared with 13 percent of those tweeting about France and 10 percent of those mentioning China. This surprisingly high figure gains even more weight when we consider the number of tweets: The Islamic State was the second most prominent "country" on Twitter during that time frame after the United States — most probably because of the group's media exposure. Truvé and his colleagues analyzed 48 million tweets since Aug. 18, which account for about 0.5 percent of all tweets.
Social media platforms
The U.S. State Department recently launched a social media campaign under the slogan "Run. Do not walk to ISIS land," which includes a graphic video and discussions on Twitter. The Islamic State has faced opposition from social media platforms in recent months, which have suspended the group's accounts and deleted its pictures and videos. Hence, the Islamic State operates through more private and even anonymous platforms, such as Ask.fm or Kik, which enable Western supporters to contact fighters in Iraq or Syria directly.
Marriage and child benefits
Marriage and child benefits seem to have a particular relevance to the Islamic State. Every fighter receives $1,200 and a furnished apartment once he marries, according to the Syrian Center for Human Rights. Furthermore, a fighter receives $50 per child and $100 per wife each month. Even welfare-focused countries such as Finland can barely catch up with such incentives (we acknowledge these are very different cases — the Islamic State sees women as the property of men).