MB, GB, or TB? Understanding Data Storage Units
A new one-terabyte drive shows up as roughly 931 gigabytes and nothing is wrong. Two different definitions of the same prefix are both in use, and the gap between them grows with every step up the scale. The Data Storage Converter handles both.
The two definitions
Computers count in powers of two, so the natural step is 1,024 - two to the tenth. The rest of measurement counts in powers of ten, where a kilo is 1,000.
Drive manufacturers use the decimal definition: a terabyte is a million million bytes. Windows uses the binary one but labels it with the decimal prefix, so it divides that number by 1,024 four times and calls the result GB. Both are telling the truth and only one of them is consistent with its own label.
The discrepancy compounds. At kilobytes it is 2.4 per cent, at megabytes 4.9, at gigabytes 7.4, and at terabytes 10 per cent. That 10 per cent is the missing 69 GB on your new drive.
The prefixes nobody uses
There is a fix, standardised in 1998. The binary units have their own names - kibibyte, mebibyte, gibibyte, tebibyte, written KiB, MiB, GiB, TiB - leaving kB, MB, GB and TB to mean the decimal values they always meant elsewhere.
It is correct, unambiguous and almost entirely ignored outside Linux and some technical documentation, mostly because nobody wants to say "gibibyte" out loud. macOS switched to decimal years ago, which is why the same drive reads as 1 TB on a Mac and 931 GB on a PC.
Bits are not bytes
A byte is eight bits, and the distinction is where internet speeds confuse everybody. Network speeds are quoted in bits per second with a lowercase b, storage in bytes with a capital B. So a 100 Mbps connection downloads at about 12.5 MB per second, not 100.
A single letter of case is the only thing distinguishing them, which is why a download looks eight times slower than the plan promised. The Bandwidth Converter deals with that one specifically.
A sense of scale
A plain text page is a couple of kilobytes. A phone photo is three to five megabytes, a RAW photo twenty to forty. A minute of 1080p video is around 100 MB; the same minute in 4K is four to five times that. An hour of streamed HD is roughly 3 GB.
Which is the practical answer to why a phone fills up: it is almost always video.
Common questions
Why does my 64 GB phone have 50 GB free?
Partly the decimal-versus-binary gap, mostly the operating system, which takes several gigabytes before you install anything.
Is a terabyte a thousand gigabytes?
In the decimal definition, yes. In the binary one it is 1,024. That is the whole problem.
What comes after a terabyte?
Petabyte, exabyte, zettabyte. A petabyte is a thousand terabytes and is the scale at which large organisations count.
How much data does a film use?
Roughly 1 to 1.5 GB an hour for HD streaming, 7 GB or more for 4K.
Need to compare two storage figures?