According to Engineering News-Record, its actually 12.5 billion cubic meters, but that may have been before the maximum water level was reduced to remove stress: http://web.archive.org/web/20030808092931/http://enr.construction.com/news/front2003/archives/030505.asp

Measurable with Google Maps - https://www.google.com/maps/place/Mosul+Dam/@36.2120055,42.2766295,210279m/data=!3m1!1e3!4m6!3m5!1s0x40087171c23c189f:0x341308438b886b05!4b1!8m2!3d36.6325324!4d42.8170276