Guide
TB vs TiB: why they aren't the same number.
A terabyte and a tebibyte sound almost identical, but they differ by nearly 10%. That difference, not a system error, is why your '1 TB' drive never shows exactly 1,000 GB in File Explorer.
Two counting systems, one "tera" prefix
The problem starts with the definition of "tera." In the decimal system (SI), which almost every drive and memory manufacturer uses, 1 terabyte (TB) equals 1,000,000,000,000 bytes — a thousand to the fourth power, just like a kilometer is a thousand meters. In the binary system, which operating systems use to report space, 1 tebibyte (TiB) equals 1,099,511,627,776 bytes — 1,024 to the fourth power, because computers address memory in powers of 2. These are different units with different names, though the industry's common mistake for years was calling what was actually GiB "GB."
The table that explains the "missing space"
| Advertised capacity | Real bytes (decimal) | What the OS shows (binary) |
|---|---|---|
| 256 GB | 256,000,000,000 bytes | ~238.4 GiB |
| 512 GB | 512,000,000,000 bytes | ~476.8 GiB |
| 1 TB | 1,000,000,000,000 bytes | ~931.3 GiB |
| 2 TB | 2,000,000,000,000 bytes | ~1,862.6 GiB |
| 4 TB | 4,000,000,000,000 bytes | ~3,725.3 GiB |
The gap runs around 7% for small capacities and closer to 9-10% for large drives. It isn't space that "disappears," nor a manufacturer discount: it's the same physical number of bytes, expressed in two different counting systems.
Why manufacturers use decimal and operating systems use binary
Storage manufacturers adopted the decimal system because it's simpler to communicate commercially and because that's how international measurement standards define it (the same logic behind a kilogram or a kilometer). Windows, on the other hand, inherits a decades-old convention where "kilobyte" meant 1,024 bytes for binary programming convenience — and it never fully migrated to the correct binary naming (KiB, MiB, GiB, TiB), so it still displays "GB" when it's actually computing GiB. macOS, since 2009, did change Finder to show capacities in the same decimal system manufacturers use, which is why a Mac shows numbers closer to the box label than a Windows PC does.
How to verify it yourself
If you want to check the math: take the advertised capacity in bytes (say, 1,000,000,000,000 for 1 TB) and divide by 1,073,741,824 (which is 1,024³, the bytes in one GiB). The result — 931.32 — is exactly the number Windows File Explorer will show, with no calculation error and no space unfairly reserved on the drive.
Verifiable sources
Decimal and binary prefixes are formally defined by the US National Institute of Standards and Technology (NIST), which documents both systems and explains the historical origin of the confusion between them.