Understanding Data Storage Units — KB, MB, GB, TB and Beyond
January 22, 2026 · 7 min read
SI vs IEC, why your hard drive shows less than advertised, and a reference table from bit to petabyte.
Why this confuses people
There are two competing definitions for data sizes, and neither side wants to give in. Hard drive makers use base-10 (SI). Operating systems use base-2 (IEC). Both are technically right.
SI: base-10
1 KB = 1,000 bytes. 1 MB = 1,000,000 bytes. 1 GB = 1,000,000,000 bytes. This is what storage manufacturers print on the box.
IEC: base-2
1 KiB = 1,024 bytes. 1 MiB = 1,048,576 bytes. 1 GiB = 1,073,741,824 bytes. Windows reports sizes this way (and mislabels them as "GB").
Why your 1 TB drive shows 931 GB
The drive really does hold 1,000,000,000,000 bytes (1 TB in SI). Windows divides that by 1024³ and calls the result "GB" — giving 931. Same data, different math.
The full scale
Bit → Byte (8 bits) → KB → MB → GB → TB → PB → EB. A photo ≈ 3 MB, an MP3 song ≈ 4 MB, a 1080p movie ≈ 2 GB, a high-end SSD = 2 TB.
Bits vs bytes
Internet speeds are in megabits per second (Mbps). File sizes are in megabytes (MB). 1 byte = 8 bits, so 100 Mbps ≈ 12.5 MB/s of actual download speed.
Frequently asked questions
Is 1 GB = 1000 MB or 1024 MB?
1 GB (SI) = 1000 MB. 1 GiB (IEC) = 1024 MiB. Both are commonly called "GB" — context tells you which.
Why does my 1 TB drive show 931 GB?
The drive holds 10¹² bytes; the OS displays it in base-2, where that equals 931 GiB.
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