WHAT IS THE USABLE CAPACITY OF MY MEMORY CARD

There is always a small difference between the memory capacity displayed on the package and the one displayed on a computer.
There are three reasons for this:

Part of the memory of the memory card is reserved by the system

This represents about 3% of the initial capacity of the memory card.

Example:
A 32 GB memory card contains 32,000,000,000 bytes
32,000,000,000 - 3% = 31,000,000,000 = 31 GB after formatting the memory card

Depending on tde type of formatting (file system) the difference will be more or less for the same memory card.
See screenshots below*

Difference in the mode of calculation between human and computer

The memory capacity of the memory cards is always indicated in the decimal system respecting the units
of the International System (SI), this applies to the packaging, product sheets, and websites of the memory cards.
1 GB = 10^9 bytes = 1 000 000 000 bytes = 1 billion bytes

However, computers do not work with the decimal system as humans do, but with the binary system.
In the binary system the unit is the GiB and not the GB.
1 GiB = 2^30 bytes = 1 073 741 824 bytes

Example:
A 32 GB memory card witd 31 GB free after formatting, or 31,000,000,000 bytes.
If we convert this value in GB: 31 000 000 000 / 1 000 000 000 = 31 GB
If we convert this value in GiB: 31 000 000 000 / 1 073 741 824 = 28.8 GiB
So 31 Gb = 28.8 Gb

Some OS display GB and otders GiB for the capacity

Warning! Windows is an OS that does not respect the International System (IS).
In Windows the value of the memory capacities is displayed in binary (GiB)
but the unit of capacity is displayed in Decimal (GB).
This unit error can mislead the unsuspecting user because it generates a capacity difference of about 7%

Windows rounds values to the tenth (one digit after the decimal point at most).
Therefore: the values displayed in Windows should be 31 GB or 28.8 GiB but not 28.8 GB*.

Under MacOS the values and the unit of the memory capacities are in decimal.
Apple respects the International System (SI).

Value table (GB vs Gib)

RAW Capacity in GB (Values displayed on packaging)	Capacity in GiB After formatting (Values displayed under Windows GB)
8 GB	7.2 GiB
16 GB	14.4 GiB
32 GB	28.8 GiB
64 GB	57.6 GiB
128 GB	115.2 GiB
256 GB	230.4 GiB
512 GB	460.8 GiB

The values below are given as an indication and may vary depending on the memory card and the type of formatting

*All the screenshots on this page were taken with tde same 32 GB SD card.