GB vs GiB: decimal and binary storage units
1 GB = 1,000,000,000 bytes; 1 GiB = 1,073,741,824 bytes. So 1 GB = 0.931323 GiB, and a 1 TB drive holds 931.323 GiB — which Windows labels "931 GB".
Two kinds of gigabyte
Storage makers, network speeds and macOS use decimal prefixes: kilo = 1,000, mega = 1,000,000, giga = 1,000,000,000. Memory chips and Windows use binary multiples: 1,024, 1,048,576 and 1,073,741,824 bytes.
The binary units have their own names — kibibyte (KiB), mebibyte (MiB), gibibyte (GiB), tebibyte (TiB) — but Windows still writes them as KB, MB and GB, which is why the same drive appears to shrink when you plug it in.
Advertised size vs what the computer shows
| Advertised | Bytes | Shown in binary units |
|---|---|---|
| 64 GB | 64,000,000,000 | 59.6046 GiB |
| 128 GB | 128,000,000,000 | 119.209 GiB |
| 256 GB | 256,000,000,000 | 238.419 GiB |
| 512 GB | 512,000,000,000 | 476.837 GiB |
| 1 TB | 1,000,000,000,000 | 931.323 GiB |
| 2 TB | 2,000,000,000,000 | 1.81899 TiB |
No space is missing
The difference is only in how the number is written. A small extra amount is also used by the file system and, on phones, by the operating system, so the free space you see is lower still — but that is separate from the GB/GiB difference.
Frequently asked questions
Is 1 GB 1,000 MB or 1,024 MB?
1 GB is 1,000 MB (decimal). 1 GiB is 1,024 MiB (binary).
Why does my 1 TB drive show 931 GB?
Because Windows shows GiB but labels it GB: 1 TB = 931.323 GiB.
Which unit does RAM use?
RAM is built in binary sizes, so "16 GB" of RAM is 16 GiB.
Open the Data Size Converter →