Why does my 1 TB drive show less space?
Drive makers and macOS count in powers of 1,000. Windows and many tools count in powers of 1,024 but still write “GB”. Convert any size and see both.
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Advertised size, three ways
| Advertised | Bytes | On a Mac (decimal) | In binary units |
|---|---|---|---|
| 256 GB | 256,000,000,000 | ≈ 256 GB | 238.42 GiB |
| 512 GB | 512,000,000,000 | ≈ 512 GB | 476.84 GiB |
| 1 TB | 1,000,000,000,000 | ≈ 1 TB | 0.91 TiB (931 GiB) |
| 2 TB | 2,000,000,000,000 | ≈ 2 TB | 1.82 TiB (1,863 GiB) |
| 4 TB | 4,000,000,000,000 | ≈ 4 TB | 3.64 TiB (3,725 GiB) |
| 8 TB | 8,000,000,000,000 | ≈ 8 TB | 7.28 TiB (7,451 GiB) |
A Mac also shows a few GB less capacity than advertised, because small hidden partitions are set aside for startup and recovery. That space isn’t lost: macOS uses it.
Decimal vs binary, in one minute
A gigabyte (GB) is 1,000,000,000 bytes. A gibibyte (GiB) is 1,073,741,824 bytes (1,024³), about 7.4% more. Since Mac OS X 10.6 Snow Leopard, macOS shows sizes in decimal units, the same way drives are sold, so a 1 TB SSD shows as about 1 TB. Windows shows the same drive as about 931 “GB” because it divides by 1,024³ but keeps the GB label.
Neither is wrong; they are two units. The difference grows with size: about 2.4% for KB, 4.9% for MB, 7.4% for GB and 10% for TB.