KB vs KiB: Why Your Drive Looks Smaller Than Advertised

Buy a "500 GB" drive, plug it in, and the computer reports about 465. Nothing is missing and nothing is broken — the two numbers are the same bytes counted in different systems, and the gap between kilobytes and kibibytes compounds at every step up the ladder.

Two counting systems, one word

Prefixes like kilo, mega, and giga come from the metric (SI) system, where each step is a clean power of 1,000. Computers, however, count naturally in powers of two, and 1,024 = 2¹⁰ is the nearest power of two to 1,000. For decades hardware and software used KB to mean 1,024 bytes anyway, and the ambiguity stuck until 1998, when the IEC introduced separate binary prefixes — kibi, mebi, gibi, tebi — for the powers of two:

SI (decimal)              IEC (binary)
1 kB  = 10³  bytes        1 KiB = 2¹⁰ = 1,024 bytes
1 MB  = 10⁶  bytes        1 MiB = 2²⁰ = 1,048,576 bytes
1 GB  = 10⁹  bytes        1 GiB = 2³⁰ = 1,073,741,824 bytes
1 TB  = 10¹² bytes        1 TiB = 2⁴⁰ = 1,099,511,627,776 bytes

So 1 KiB is always 1,024 bytes — which is 1.024 kB, slightly more than a kilobyte. The words look interchangeable and are not.

Why the gap compounds: 2.4% per step

Each binary step is 1.024 times its decimal counterpart, so the mismatch grows multiplicatively: 2.4% at kilo, 4.9% at mega, 7.4% at giga, and 9.1% at tera — because (1024/1000)³ ≈ 1.0737 and (1024/1000)⁴ ≈ 1.0995. This is why the discrepancy is invisible at file sizes (a "100 KB" file is off by 2 KB either way) and glaring at drive sizes, where two seven-percent-scale numbers meet.

The missing gigabytes, worked out

A drive sold as 500 GB genuinely holds 500,000,000,000 bytes — the manufacturer is using the SI prefix correctly. The operating system then divides by the binary unit: 500,000,000,000 / 1,073,741,824 = 465.66 GiB. Windows displays that as "465 GB" — it is really reporting GiB under the old label, which is where the sense of theft comes from. Divide the same bytes by SI's 10⁹ and you get exactly 500 GB. Also worth knowing: a "64 GB" phone that shows 59 is losing a couple of GiB to the same conversion plus the operating system's own partition. There is a famous oddball, too — the "1.44 MB" floppy disk was actually 1,440 KiB = 1,474,560 bytes, matching neither convention, which tells you how loose the old usage was.

Who uses which convention

• Decimal (SI): drive and SSD marketing, network speeds (1 Mbps = 1,000,000 bits per second), and macOS file sizes, which switched to decimal in 2009.
• Binary (IEC or unlabeled): RAM and CPU cache (memory addressing is inherently binary — a chip cannot have 1,000,000,000 addressable cells), and Windows file listings, which show binary sizes with SI labels.
• Either, unhelpfully: download progress bars, game requirements, and cloud storage quotas — check which side a figure actually means before comparing.

Linux distributions split the difference: most now print "1.0 GiB" and "1.0 GB" as genuinely different numbers in the same listing, which is exactly right.

Converting without ambiguity

The reliable method is to convert to bytes first, then to the target unit — the same pass-through-one-base trick every unit converter uses. The subtlety is choosing which convention the number in front of you follows. Our two converters take opposite sides deliberately: the Data Unit Converter uses strict modern labels, where KB is 1,000 bytes and KiB is 1,024, so a conversion answers in the unit you asked for; the Data Size Converter follows the traditional KB = 1,024 convention that file managers and memory specs still use. If a figure came from a drive box, treat it as decimal; if it came from a file listing or RAM spec, treat it as binary. And because the binary ladder is built on powers of two, the underlying arithmetic is the same positional logic described in number base systems — 1 GiB is 2³⁰ bytes, full stop.

Try it yourself

Type your drive's advertised size into the Data Unit Converter and read the GiB column — that is the number your file manager will show. Convert the same figure with the Data Size Converter to see the traditional 1024-style interpretation, and use the Binary Converter to explore the powers of two the binary ladder is built on.