Understanding data size
Megabyte vs mebibyte.
The reason your "1 TB" disk shows up as 931 GB. It isn't a lie — it's two definitions colliding.
Bits and bytes.
The bit is the atom of digital information — a single binary choice, 0 or 1. Eight bits make a byte, the smallest unit most systems can address directly. Networking still talks in bits per second; storage and memory speak almost entirely in bytes.
1 byte = 8 bits
The decimal ladder.
Above the byte, the SI prefixes apply: every step is a thousand, just like grams and metres. A megabyte is a million bytes, a gigabyte is a billion. This is what disk manufacturers, network providers, and the SI itself use.
The binary ladder.
Operating systems prefer powers of two — 1024 of one thing per one of the next, because 2¹⁰ is a more natural step than 10³ in digital hardware. To resolve the ambiguity, in 1998 the IEC codified a separate set of prefixes (KiB, MiB, GiB) for the binary versions. Most operating systems use these binary values but display the decimal symbols, which is where the confusion comes from.
Practical equivalences
- 1 KiB = 1024 B = 1.024 KB
- 1 MiB = 1 048 576 B ≈ 1.049 MB
- 1 GiB = 1 073 741 824 B ≈ 1.074 GB
- 1 TB (decimal) ≈ 0.909 TiB
Three worked conversions.
1 GiB to MB
1 GiB = 1 073 741 824 B · 1 MB = 1 000 000 B
Convert through the byte.
1 073 741 824 ÷ 1 000 000 ≈ 1073.74
= 1073.74 MB
500 MB to GB
1 GB = 1000 MB
Both are decimal; divide by a thousand.
500 ÷ 1000 = 0.5
= 0.5 GB
100 megabits to megabytes
1 byte = 8 bits
Connection speeds in Mb/s are in bits, file sizes in MB are in bytes — divide by eight.
100 ÷ 8 = 12.5
= 12.5 MB
Why your terabyte drive isn't a terabyte.
The drive vendor used decimal: 1 TB = 10¹² bytes. The operating system reads it in binary and reports 10¹² ÷ 2⁴⁰ ≈ 0.909 TiB, rounded to 931 GiB and labelled GB. Both numbers are correct; they're answering different questions. The honest fix would be for operating systems to display "GiB" — which is what some (Linux file managers, recent macOS) now do.
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