My confusion about storage units went on longer than it should have — I kept dismissing the gigabyte vs gibibyte distinction as pedantic until a drive reporting less space than advertised made me look properly.
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 file size calculator converts between all decimal and binary units — bytes, KB, KiB, MB, MiB, GB, GiB, TB, and TiB — and shows whether a file fits on common storage media from a floppy disk to a 1 TB SSD. Our data storage converter also handles these conversions. 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.
Why the Numbers Never Add Up the Way You Expect
The confusion runs deeper than most people realise because it exists at multiple levels simultaneously. When a hard drive manufacturer labels a drive as 1 TB, they mean 1,000,000,000,000 bytes — which is mathematically correct by the SI definition of "tera." But your operating system reports capacity in tebibytes (TiB), where 1 TiB equals 1,099,511,627,776 bytes. The result is that a 1 TB drive shows up as roughly 931 GiB in Windows or 909 GiB on some Linux systems — and that gap is not missing space, it's a labelling mismatch that's been baked into the industry for decades.
The Full Hierarchy of Storage Units
Understanding the complete chain of units helps once you can see it laid out clearly. Starting from bytes: 1 KB (kilobyte) = 1,000 bytes, while 1 KiB (kibibyte) = 1,024 bytes. Scale that up: 1 MB = 1,000,000 bytes versus 1 MiB = 1,048,576 bytes. By the time you reach gigabytes, the gap is 73.7 MB per unit. At terabytes, you're looking at a difference of roughly 73.7 GB — which is why a 1 TB drive appears to have "lost" nearly 10% of its capacity when the OS reports it in base-2 units. The International Electrotechnical Commission standardised the binary prefixes (KiB, MiB, GiB, TiB) in 1998 specifically to resolve this, but the industry has been slow to adopt consistent labelling.
Where This Actually Matters in Practice
The confusion creates real problems in several common situations. Cloud storage pricing is one: some providers charge in GB (base-10) while usage is reported in GiB (base-2), so your bills can look inconsistent even when you're using the amount you expected. File transfer estimates are another: download managers that calculate time based on MB/s often mix the two standards depending on what the speed figure represents. Server provisioning matters too — allocating what you think is 500 GB of storage may result in a partition that appears smaller depending on how the filesystem reports it. For video production, the gap between GB and GiB frequently causes confusion when estimating how many hours of footage a drive will hold.
A Practical Reference for Quick Conversions
If you need a fast mental model: multiply GB by 0.931 to get approximate GiB (so 500 GB ≈ 466 GiB). Going the other direction, multiply GiB by 1.074 to get approximate GB. For TB to TiB, multiply by 0.909. These are rounded figures that work well for everyday estimates — for precision, use the calculator which handles the exact binary arithmetic. The important habit to build is checking which unit a figure is using before comparing two numbers. A 500 GB SSD and a 500 GiB SSD are not the same size, and confusing them when buying, provisioning, or planning backups creates downstream problems that take time to untangle.