BBC - In September 2014, the then director of the US National Counterterrorism Center, Matthew Olsen, said IS controlled much of the Tigris-Euphrates river basin - an area similar in size to the United Kingdom, or about 210,000 sq km (81,000 sq miles).
A year later, the US defense department declared that IS front lines in much of northern and central Iraq and northern Syria had been pushed back significantly by US-led coalition air strikes and ground operations. IS could no longer operate freely in roughly 20-25% of populated areas in Iraq and Syria where it once could, it said.
The defense department estimated that IS had lost approximately 5,790-7,720 sq miles of territory in Iraq, or about 30-37% of what it controlled in August 2014, and 770-1,540 sq miles)in Syria, or about 5-10%.
Despite this, IS has been able to capture new territory of strategic value over the same period, including the city of Ramadi in Iraq's Anbar province and Palmyra in Syria's Homs province.
Analysts also note that the US figures do not necessarily reflect the situation on the ground. In reality, IS militants exercise complete control over only a small part of that territory, which includes cities and towns, main roads, oil fields and military facilities.
[ISIS now controls land approximately the size of South Dakota, the 16th largest state in the union - TPR]
InFebruary 2015, US Director for National Intelligence James Clapper said IS could muster "somewhere in the range between 20,000 and 32,000 fighters" in Iraq and Syria.
But he noted that there had been "substantial attrition" in its ranks since US-led coalition air strikes began in August 2014. In June 2015, US Deputy Secretary of State Antony Blinken said more than 10,000 IS fighters had been killed.
To help mitigate the manpower losses, IS has turned to conscription in some areas. Iraqi expert Hisham al-Hashimi believes only 30% of the group's fighters are "ideologues", with the remainder joining out of fear or coercion.
[ISIS is roughly the size of one American Army division. The Army has ten such divisions, the Marines three- TPR]