How many GB is 612.3mb

Assuming mb is megabits and GB is gigabytes:

612.3 Mb (in bits) = 612.3 * 1024 * 1024 = 642,043,084.8 b
1 GB (in bits) = 1 * 8 * 1024 * 1024 * 1024 = 8,589,934,592 b

8,589,934,592 / 642,043,084.8 = 13.079 (to 3 d.p.).