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 1 GB = 1.00 Billion bytes.
Please use high quality USB 3.0 card reader to test all memory cards. We recommend using H2testw app on PC.
Please don't use cheap card reader to test Micro SD card. Speed of Micro SD card will be reduced by restrictions of the low quality card reader.
The most reliable software to test the memory card is H2testw.
SanDisk Micro SD with Retail Package x 1