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MB vs MiB: Why Your 1 TB Drive Shows Only 931 GB

By Dvir ·

You buy a 1 TB drive, plug it in, and Windows reports 931 GB. No, the store didn’t short-change you, and no, the file system didn’t eat 69 gigabytes (formatting overhead is real but tiny by comparison). What you’re seeing is two industries using the same words for two different numbers — and once you see the trick, you’ll never be confused by a storage spec again.

Two definitions of “kilo”

In the metric system, kilo means 1,000. A kilometer is 1,000 meters, a kilogram is 1,000 grams. When storage vendors say kilobyte (KB), megabyte (MB), gigabyte (GB) or terabyte (TB), they mean exactly that: powers of 1,000.

  • 1 KB = 1,000 bytes
  • 1 MB = 1,000,000 bytes
  • 1 GB = 1,000,000,000 bytes
  • 1 TB = 1,000,000,000,000 bytes

Computers, though, are built on binary, and powers of two are natural to them. Early programmers noticed that 2¹⁰ = 1,024 is almost 1,000 and started calling 1,024 bytes a “kilobyte” too. That shortcut worked fine when the difference was 2.4% and drives were tiny. It stopped being fine when the units stacked up.

To fix the ambiguity, the IEC standardized binary prefixes in 1998:

  • 1 KiB (kibibyte) = 1,024 bytes
  • 1 MiB (mebibyte) = 1,024² = 1,048,576 bytes
  • 1 GiB (gibibyte) = 1,024³ = 1,073,741,824 bytes
  • 1 TiB (tebibyte) = 1,024⁴ = 1,099,511,627,776 bytes

The “i” in the middle is the tell: MiB is binary, MB is decimal. They are different sizes, and the gap grows with every step up the ladder — 2.4% at kilo, 4.9% at mega, 7.4% at giga, and 10% at tera.

Where the 69 GB “goes”

Now the drive mystery solves itself. The manufacturer counts your 1 TB drive in decimal: 1,000,000,000,000 bytes. That’s honest and standards-compliant. Windows then takes those same bytes and divides by 1,024³ to display the size — but labels the result “GB” when it’s really GiB:

1,000,000,000,000 ÷ 1,073,741,824 ≈ 931 GiB

Same bytes, different ruler. Nothing is missing. You can check the math yourself with the GB to GiB converter, or see the full-size version of the effect with TB to TiB.

The confusion is entirely a labeling problem. macOS switched to true decimal units back in 2009, which is why the same drive shows “1 TB” on a Mac and “931 GB” on Windows. Linux tools mostly use proper IEC notation (GiB), which is the most honest of the three.

Where each system shows up

Knowing which convention you’re looking at is half the battle:

Decimal (MB, GB, TB — powers of 1,000):

  • Hard drive and SSD capacities on the box
  • Network speeds (a “100 Mbps” line is 100,000,000 bits per second)
  • DVD capacities, cloud storage plans, phone storage marketing

Binary (MiB, GiB, TiB — powers of 1,024):

  • RAM (“16 GB” of memory is really 16 GiB — memory is the one place vendors use binary sizes)
  • Windows file and drive sizes (labeled GB, measured GiB)
  • Linux command-line tools, cloud instance specs (an AWS instance’s “8 GiB” is labeled correctly)

Yes, that means a “16 GB” RAM stick and a “16 GB” USB drive hold different numbers of bytes. The RAM holds about 7.4% more.

Bits are a third trap

While we’re here: a byte is 8 bits, and network speeds are quoted in bits per second. A “500 Mbps” internet plan moves at most 500 ÷ 8 = 62.5 MB of file data per second — before overhead. When a 5 GB download crawls along at “60 MB/s” on a “500 meg” connection, everything is working exactly as advertised; the units just switched from bits to bytes between the ad and the download dialog. The bits to bytes converter makes that eight-fold gap concrete.

Rules of thumb worth keeping

  1. Drives look smaller in Windows by ~7–10%. A 2 TB drive shows ≈1.81 TiB (labeled TB). That’s the definition gap, not a defect.
  2. The gap compounds with size. KB→KiB differs by 2.4%, but TB→TiB by 10%. The bigger the drive, the bigger the apparent shrinkage.
  3. Divide network speeds by 8. Mbps to MB/s is the single most useful storage conversion in daily life.
  4. Watch for the “i”. MiB, GiB and TiB are binary; MB, GB and TB are (supposed to be) decimal. When a program omits the “i” you have to guess from context — Windows means binary, drive vendors mean decimal.
  5. Formatting overhead is separate and small. File system structures typically cost a percent or two, on top of the unit difference.

Converting between them

The conversion factors aren’t hard, but they’re not clean either — 1 GB is 0.9313 GiB, 1 MiB is 1.048576 MB — so this is exactly the kind of arithmetic worth delegating. The data storage converter handles all twelve units (bits and bytes through PB and PiB, decimal and binary) in one place, and pages like MB to MiB show the factor, a worked example and a reference table for the specific pair you’re staring at.

Once you internalize that there are simply two rulers — 1,000 for marketing and networks, 1,024 for operating systems and memory — every “missing storage” mystery becomes a ten-second conversion instead of a support ticket.