this the explination on the sandisk knowledebase. i underlined the specific explination for the capacity difference. This is common among all flash memory manufacturers.
Why is the capacity of my device (as reported by many operating systems) different than the capacity that is listed on its label?
All SanDisk products include the total capacity, at an unformatted level, that is stated on the product packaging. For example, a 128GB SanDisk USB flash drive has a total capacity of 128,000,000,000 bytes at the unformatted level (where 1GB=1,000,000,000 bytes).
Definitions of a Megabyte :
1. Operating Systems commonly define capacity as follows:
- Kilobyte (KB) as: 2 to the 10th power (1,024 bytes)
- Megabyte (MB) as: 2 to the 20th power (1,024 X 1,024 bytes = 1,048,576 bytes)
- Gigabyte (GB) as: 2 to the 30th power (1,024 X 1,024 X 1,024 bytes = 1,073,741,824 bytes)
2. Disk Drive and Flash Memory Card Manufacturers commonly define a MB as one million bytes (exactly 1,000,000 bytes) and a GB as one billion bytes (exactly 1,000,000,000 bytes).
SanDisk defines 1 GB as 1,000,000,000 bytes. Operating Systems define 1 GB as 1,073,741,824 BYTES.
Note: Some capacity is used for formatting and other functions and thus not available for data storage.
A portion of the total capacity is used to store certain functions including optimizations of the memory that support performance and endurance and therefore is not available for user storage. This is disclosed on our packaging and marketing materials when you see the statement “Actual user storage less.”
As a leader in flash storage technology, SanDisk offers multiple products at different speeds and capacities to address a variety of consumer needs. To provide the optimum product for you, we’re continuously improving and updating our technology, products, firmware, and design. Continual improvements and updates are made to our product (both new products and existing product lines) and can include changes to the basic file structure, formatting and other functions. These optimizations may result in variances as to the amount of memory space that is available for user storage, but are essential to performance and endurance of the products.