Security and defence analysts IHS say the group's control has shrunk by 28% since its height in January 2015.
In the first nine months of this year, IS' territory fell from 78,000 sq km (30,115 sq miles) to 65,500 sq km - an area equivalent to the size of Sri Lanka - IHS analysts said.
However, IS losses have slowed in the three months to October.
IS has lost just 2,800 sq km (1,080 sq miles) since July.
The slowdown appears to coincide with Russia reducing the number of air strikes against IS targets, IHS has observed.