KB vs KiB: Why Your Drive Shows Up Smaller Than the Box Says

2026-09-20 · 2 min read

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A drive advertised as "1 TB" shows up in your file manager as "931 GB." Nothing was lost and nobody lied — two different definitions of "kilo" are both being called "GB," and the gap between them is one division that gets bigger the further up the scale you go.

The method: decimal units and binary units are both fixed byte counts

Storage is sold in decimal units, where each step multiplies by 1,000: 1 kilobyte (KB) is 1,000 bytes, 1 megabyte (MB) is 1,000,000 bytes, 1 gigabyte (GB) is 1,000,000,000 bytes, 1 terabyte (TB) is 1,000,000,000,000 bytes. Operating systems typically display binary units instead, where each step multiplies by 1,024: 1 kibibyte (KiB) is 1,024 bytes, 1 mebibyte (MiB) is 1,048,576 bytes, 1 gibibyte (GiB) is 1,073,741,824 bytes. Converting between any two of these — decimal to decimal, binary to binary, or across the two families — is the same two steps as any unit conversion: multiply your value by the source unit's byte count, then divide by the target unit's byte count.

Worked example: why "1 TB" reads as "931 GB"

A 1 TB drive holds 1,000,000,000,000 bytes — the correct, honest decimal count, and what you paid for. The file manager's "GB" column, though, is usually counting in binary GiB. Converting: 1,000,000,000,000 bytes ÷ 1,073,741,824 bytes per GiB = 931.32. The physical capacity didn't change; the label on the column did.

The same division explains a 500 GB drive reading as 465.66 GiB (500,000,000,000 ÷ 1,073,741,824), and an "8 GB" RAM stick reading as roughly 7.45 GiB of the same physical chips (8,000,000,000 ÷ 1,073,741,824) — before the operating system even sets aside any of that for itself, which is a separate loss on top of the unit conversion, not part of it.

The gap widens as the units get bigger

Because 1,000 and 1,024 are close but not equal, a decimal unit is always a bit smaller than the same-named binary one, and the shortfall compounds one more time at every step up the scale: a KB is 2.34% smaller than a KiB, an MB is 4.63% smaller than a MiB, a GB is 6.87% smaller than a GiB, and a TB is 9.05% smaller than a TiB. That's why the "missing space" complaint gets louder the bigger the drive: at kilobyte scale the gap is barely noticeable, and at terabyte scale it's nearly a tenth of the number.

The other direction: bytes and bits

The same conversion table includes bits, at exactly 1/8 of a byte. A 32-bit number is 32 × 0.125 = 4 bytes; a 64-bit one is 64 × 0.125 = 8 bytes — the identical multiply-then-divide method, just with a factor smaller than one. (Network speeds mix bits and bytes even more directly than storage does; that's the subject of the companion post on data-rate units.)

What the tool adds

The byte counts above are everything the method needs on paper. The free Data Size Converter tool runs the same multiplication and division instantly across bytes, KB/MB/GB/TB, KiB/MiB/GiB and bits, so you never have to remember which "kilo" a given number meant — entirely on your device.