ISIS has a remarkably simple way of calculating taxes

ISIS (also known as ISIL, Islamic State, and Daesh) controls a land mass the size of Britain and rakes in so much revenue from oil, tax and confiscation that it is unlike any other Islamist terrorist group in history.

While most terrorist groups, like Al Qaeda, rely on donations for funding its operations, ISIS is self-sufficient.

One of the keys ways in which is does that is by taxation. In fact, according a fantastic in-depth report by the Financial Times, ISIS "earns at least as much from taxation, extortion and confiscation as oil."

And ISIS has a remarkably simple way in calculating and collecting taxes.

Firstly, its tax or "zakat" dates from the times of Prophet Mohammed in the 7th century. Zakat is a form of almsgiving where citizens of an Islamist state would give 2.5% of their capital towards authorities to fight for a "holy cause."

Secondly, when it comes to taxing goods, ISIS tax collectors - which work for the local Zakat Council under a provincial "wali" (governor) - calculate how much goods are worth by physically going round to stores to make a market comparison.

The FT gave a first hand account of how Mansour (not his real name), a 26-year-old grocery storekeeper in eastern Syria fell foul of tax collectors when he tried to stall his zakat payments:

A week later, four Isis officials stormed into his shop, ordered him outside, and tallied the bill themselves - to his dismay they based their calculation on the retail price of his stock. There were no price tags on the tinned beef, so one tax collector rode around town on his motorbike comparing canned beef prices in other stores.

Five hours later, the audit was complete. The bill: 32,500 Syrian lira (about $108).

"They told me, 'You liar?.?.?.?How will victory be achieved if you're not paying zakat?'"

At the beginning of December IHS research estimated that ISIS rakes in $80 million (£53 million) per month. Around $1.1 million (£730,793) per day is made from just oil revenue alone.

As Oscar Williams-Grut reported this month, ISIS gets all of its funding from the following ways:

• Scamming banks
• Selling antiques and artifacts
• Ransoms
• Oil
• Taxation
• Donations

Western forces are concentrating on bombing oil fields and trucks in order to stop a massive flow of funding into ISIS. However, the FT reports that "over the past year, taxation and confiscation probably rivalled oil as the group's main source of revenue."