Digital storage has two competing definitions, and almost every confusing number you have seen about drive capacity comes from the gap between them. Storage manufacturers use decimal units: one gigabyte means exactly 1,000,000,000 bytes. Operating systems — Windows most visibly — report in binary units, where the same quantity is divided by 1,073,741,824. The drive did not shrink; it is being measured with a different ruler.
That is why a "1 TB" drive shows up as roughly 931 GB in Windows. The gap widens as capacity grows: about 2.4% at the kilobyte level, 7.4% at gigabytes, and nearly 10% at terabytes.
Decimal versus binary units
| Decimal (SI) | Bytes | Binary (IEC) | Bytes |
|---|---|---|---|
| Kilobyte (kB) | 1,000 | Kibibyte (KiB) | 1,024 |
| Megabyte (MB) | 1,000² | Mebibyte (MiB) | 1,024² |
| Gigabyte (GB) | 1,000³ | Gibibyte (GiB) | 1,024³ |
| Terabyte (TB) | 1,000⁴ | Tebibyte (TiB) | 1,024⁴ |
| Petabyte (PB) | 1,000⁵ | Pebibyte (PiB) | 1,024⁵ |
Bits versus bytes
A byte is eight bits, and the distinction matters most when networks are involved. Internet connections are sold in megabits per second (Mbps); files are measured in megabytes (MB). Divide by eight to convert: a 100 Mbps connection moves at most about 12.5 MB per second, so a 5 GB download takes roughly seven minutes at full speed, not fifty seconds. Lowercase "b" means bits, uppercase "B" means bytes — a convention that is easy to miss and accounts for a great deal of disappointment with broadband speeds.
Rough sizes for common files
- Plain text page: 2–5 kB — a whole novel is typically under 1 MB.
- Smartphone photo: 2–5 MB for JPEG, 15–30 MB for RAW.
- MP3 audio: about 1 MB per minute at 128 kbps.
- Streaming video: roughly 1 GB per hour at 1080p, 7 GB per hour at 4K.
- Modern game install: 50–150 GB, which is why storage planning is mostly a games question.
Why usable capacity is always less than advertised
Beyond the decimal-versus-binary gap, some capacity is genuinely unavailable. The file system consumes space for its own metadata, SSDs reserve a hidden pool for wear levelling and bad-block replacement, and a factory recovery partition can take tens of gigabytes. Budget around 10% below the label for a hard drive and slightly more for an SSD, and you will not be caught short.
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