Buy a "1 TB" hard drive, plug it in, and your computer will almost certainly report something like 931 GB available — not a defect, not a scam, just two different definitions of the word "gigabyte" colliding. Storage manufacturers and operating systems have quietly disagreed about what a kilobyte means for decades, and the gap grows larger as drives get bigger.
Drive manufacturers label capacity in decimal — the same base-10 system as every other metric unit, where each prefix is a clean multiple of 1,000:
| Unit | Decimal value |
|---|---|
| 1 kilobyte (kB) | 1,000 bytes |
| 1 megabyte (MB) | 1,000,000 bytes |
| 1 gigabyte (GB) | 1,000,000,000 bytes |
| 1 terabyte (TB) | 1,000,000,000,000 bytes |
Operating systems, on the other hand, historically report capacity in binary — powers of 2, because that's how computers actually address memory internally:
| Unit | Binary value |
|---|---|
| 1 kibibyte (KiB) | 1,024 bytes (2¹⁰) |
| 1 mebibyte (MiB) | 1,048,576 bytes (2²⁰) |
| 1 gibibyte (GiB) | 1,073,741,824 bytes (2³⁰) |
| 1 tebibyte (TiB) | 1,099,511,627,776 bytes (2⁴⁰) |
The catch: Windows (and older macOS) label the binary value with the decimal name — showing "GB" on screen when it's actually calculating in GiB. Neither number is wrong; they're just answering different questions using the same word.
A 500 GB drive, as labeled by the manufacturer, contains exactly 500,000,000,000 bytes. Convert that into the binary units your OS actually displays:
Windows shows this as "465 GB" — technically mislabeled, since it's really 465.7 GiB, but the number itself is accurate. Nothing is missing from the drive; it's genuinely all there, just measured on a different scale than the box advertised.
| Advertised (decimal) | Shown in Windows (binary) |
|---|---|
| 128 GB | ≈119 GB |
| 256 GB | ≈238 GB |
| 500 GB | ≈466 GB |
| 1 TB | ≈931 GB |
| 2 TB | ≈1.82 TB |
| 4 TB | ≈3.64 TB |
| 10 TB | ≈9.09 TB |
This isn't a modern trick — it predates gigabyte-scale drives entirely, and both sides have a reasonable case. Manufacturers use decimal because it's the standard SI convention used for every other physical measurement (kilometers, kilograms, and so on), and it's also, not incidentally, the larger-looking number. Operating systems historically used binary because computer memory is inherently organized in powers of two at the hardware level, making binary math the natural unit for how a filesystem actually addresses space.
In 1998, the International Electrotechnical Commission introduced kibi-, mebi-, gibi-, and tebi- as distinct, unambiguous binary prefixes specifically to end this confusion. Adoption has been slow — Windows and Linux still often display "GB" on screen when they mean GiB — but macOS and some Linux distributions have since switched to reporting true decimal values, meaning a "500 GB" drive shows closer to the full 500 GB on those systems instead of 465.
For any specific figure, the Digital Storage category on the UnitDial converter handles bit, byte, kilobyte, megabyte, gigabyte, and terabyte conversions directly, using the standard decimal definitions.