You buy a 1TB hard drive. Plug it in. Windows reports 931GB. You feel short-changed. You haven't been — what's happened is a collision between two different definitions of "gigabyte" that has been confusing consumers for decades. Here's the clear explanation.
The Two Definitions
Decimal (used by drive manufacturers — SI standard):
1 KB = 1,000 bytes | 1 MB = 1,000,000 bytes | 1 GB = 1,000,000,000 bytes | 1 TB = 1,000,000,000,000 bytes
Binary (used by operating systems — IEC standard):
1 KiB = 1,024 bytes | 1 MiB = 1,048,576 bytes | 1 GiB = 1,073,741,824 bytes | 1 TiB = 1,099,511,627,776 bytes
Our data storage converter converts between all these units. The crux: manufacturers label products using decimal GB/TB (correctly, per SI). Operating systems display storage in binary GiB/TiB but historically label them "GB/TB". Result: apparent shrinkage that is actually just a labelling inconsistency.
The Maths Behind "Missing" Space
1TB = 1,000,000,000,000 bytes. ÷ 1,073,741,824 bytes/GiB = 931.32 GiB. Windows displays this as "931 GB". No bytes are missing — it's the correct byte count expressed in binary units, mislabelled with the wrong prefix symbol.
Why Did This Happen?
Early computer engineers used "kilobyte" to mean 1,024 bytes (the nearest round power of 2 to 1,000 — convenient for hardware architecture). "Kilo" technically means exactly 1,000 in SI units. The IEC standardised kibibyte/mebibyte/gibibyte in 1998 to disambiguate, but operating systems are slow adopters.
Current State
macOS switched to decimal GB reporting in 2009 — its numbers match drive labelling. Windows still reports in binary values labelled as "GB". Linux varies by distribution. Neither approach is "wrong" — they're different conventions applied inconsistently.
Bits vs Bytes: The Transfer Speed Trap
Internet speeds are quoted in bits per second. File sizes are in bytes. 8 bits = 1 byte. A 100 Mbps connection transfers 12.5 MB/s — not 100 MB/s. To convert Mbps to MB/s: divide by 8. This is why your "100Mbps" connection downloads files slower than the number implies when you're thinking in bytes.
Further reading: HowToGeek has an accurate, accessible explainer on binary vs decimal storage units. Read HowToGeek's GB vs GiB explanation.