it should say in your owners manual. if not call the BMW dealer ship they will tell you. but from seeing other BMWs i would say 13 to 16 gallons maybe smaller but look in your owners manual.
The BMW 323 i takes Premium gas,but can use mid grade as well. premium is preferred.
How do you oopen the gas tank on a BMW 528i
The gas tank on the 2014 BMW 7-Series holds 21.1 gal..
The gas tank on the 2013 BMW X5-M holds 22.4 gal..
The gas tank on the 2009 BMW M6 holds 18.5 gal..
The gas tank on the 2008 BMW M6 holds 18.5 gal..
The gas tank on the 2006 BMW M3 holds 16.6 gal..
The gas tank on the 2013 BMW M6 holds 21.1 gal..
The gas tank on the 2008 BMW M5 holds 18.5 gal..
The gas tank on the 2010 BMW X6 holds 22.5 gal..
The gas tank on the 2010 BMW M5 holds 18.5 gal..