16GB = approximately 14GB-15GB
32GB = approximately 28GB-30GB
64GB = approximately 58GB-60GB
128GB = approximately 115GB-120GB
256GB = approximately 224GB-240GB
This is calculation difference between manufacturer and your PC, please Google 'Memory card capacity' to get more information.
Your PC says 1GB = 1.073741824 Billion bytes where Market Defines 1GB = 1.00 Billion bytes.