What is the usable capacity of my SSD
There is always a small difference between the memory capacity displayed on the package and the one displayed on a computer. There are two reasons for this:
I. Une Part of the memory of the USB stick is reserved by the system for its own operation.
Depending on the type of formatting (file system) the difference will be more or less for the same USB stick. See screenshots below*
II. Difference in the mode of calculation between human and computer :
The memory capacity of the USB sticks 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 USB sticks.
1 GB = 109 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 = 230 bytes = 1 073 741 824 bytes
Example:
A 32 GB USB flash drive with 31 GB free after formatting, or 31,000,000,000 bytes.
If we convert this value in GB: 512 000 000 000 / 1 000 000 000 = 512 GB
If we convert this value in GiB: 512 000 000 000 / 1 073 741 824 = 476 GiB
So 512 GB = 476 GiB
III. To display the capacities of the USB flash drives some OS (Operating System) display GB and others GiB.
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 512 GB or 476 GiB but not 476 GB*.
Under MacOS the values and the unit of the memory capacities are in decimal. Apple respects the International System (SI).
The value displayed in MacOS is 512.11 GB*
IV. Tableau de valeur
| Gross Capacity in GB ( Values displayed on packaging) | Capacity in GiB After formatting (Values displayed under Windows GB) |
|---|---|
| 120 GB | 112 GiB |
| 128 GB | 120 GiB |
| 240 GB | 223 GiB |
| 256 GB | 238 GiB |
| 480 GB | 447 GiB |
| 500 GB | 465 GiB |
| 512 GB | 476 GiB |
| 1000 GB | 930 GiB |
| 1024 GB | 952 GiB |
| 2000 GB | 1860 GiB |
| 2048 GB | 1905 GiB |
| 4000 GB | 3720 GiB |
| 4098 GB | 3809 GiB |
The values below are given as an indication and may vary depending on the USB key and the type of formatting
*All the screenshots on this page were taken with the same 512 GB external SSD.