Two meanings of "gigabyte"
- Decimal (GB): 1 GB = 1,000,000,000 bytes. Drive makers, phone makers and macOS use this.
- Binary (GiB): 1 GiB = 1,073,741,824 bytes (1024 × 1024 × 1024). Windows counts this way but labels it "GB".
Why your drive looks smaller
A 1 TB drive holds 1,000,000,000,000 bytes. Divided by 1,073,741,824, that is about 931 binary gigabytes, so Windows shows "931 GB". Nothing is missing: the same number of bytes is being counted in a different unit. On a Mac, the same drive shows as 1 TB.
How big the gap gets
| Advertised | Shown by Windows |
| 128 GB | 119 GB |
| 256 GB | 238 GB |
| 512 GB | 476 GB |
| 1 TB | 931 GB |
| 2 TB | 1.81 TB |
Bits and bytes
Internet speeds are quoted in bits per second. There are 8 bits in a byte, so a 100 Mbps connection downloads at most 12.5 MB per second.
Convert any size
The Data Size Converter converts between bits, bytes, KB, MB, GB, TB and their binary KiB, MiB, GiB and TiB versions.
Worked examples
- A 256 GB phone or drive: 256,000,000,000 ÷ 1,073,741,824 = 238.4, shown as 238 GB by Windows.
- A 100 MB file: 100,000,000 ÷ 1,048,576 = 95.4, so the same file is "100 MB" on a Mac and "95.4 MB" in Windows.
- A 4.7 GB download on a 100 Mbps connection: 4.7 × 1,000 × 8 = 37,600 megabits, and 37,600 ÷ 100 = 376 seconds, a little over 6 minutes at full speed. The Download Time Calculator does this for any size.
Where else the space goes
The unit is not the only reason a new device shows less free space than advertised. The operating system, preinstalled apps and recovery files take their share before you save anything, often 10–20 GB on a phone or laptop.
Memory is always binary
RAM is built in powers of two, so "16 GB" of memory really is 16 GiB, 17,179,869,184 bytes, on every system.