MB vs. MiB: Why Your 1TB Drive Shows Less Than 1TB

Buy a “1TB” hard drive, plug it in, and your operating system might report something like 931 GB of usable space. No data is missing and nothing is broken — this is the result of two different definitions of “kilo,” “mega,” and “giga” being used side by side, one by storage manufacturers and one (often) by operating systems.

Two different definitions of the same prefixes

• Decimal (SI) units — kB, MB, GB, TB — use powers of 1,000: 1 MB = 1,000,000 bytes, 1 GB = 1,000,000,000 bytes. This is consistent with every other use of “kilo,” “mega,” and “giga” in the metric system (a kilometer is 1,000 meters, not 1,024 meters).
• Binary units — KiB, MiB, GiB, TiB — use powers of 1,024 (2¹⁰): 1 MiB = 1,048,576 bytes, 1 GiB = 1,073,741,824 bytes. These “kibi/mebi/gibi” prefixes were standardized by the IEC specifically to remove ambiguity from computing’s historical habit of using “kilo” to mean 1,024.

The core problem is historical: for decades, “1 KB” in computing informally meant 1,024 bytes (because 1,024 is a convenient power of 2 for memory addressing), even though “kilo” in every other context means 1,000. The IEC binary prefixes (KiB, MiB, GiB) were introduced in 1998 to give the 1,024-based units their own unambiguous names — but adoption has been inconsistent.

Why your drive “loses” capacity

Storage manufacturers label capacity using the decimal definition (1 TB = 1,000,000,000,000 bytes) — which is also the IEC/SI-correct usage of “tera.” But many operating systems display storage capacity using binary math while still showing the label “GB” or “TB” instead of the correct “GiB”/“TiB.”

So a drive marketed as “1 TB” contains exactly 1,000,000,000,000 bytes. An operating system that divides this by 1,024³ (≈ 1.0995 × 10¹²) to display the size — but labels the result “GB” instead of “GiB” — reports:

1,000,000,000,000 ÷ 1,073,741,824 = 931.3 (labeled “GB,” but is actually GiB)

This is why a “1 TB” drive shows as “931 GB” in some operating systems’ file managers — the drive manufacturer used decimal units correctly, and the OS performed a binary calculation but used the decimal unit’s name. The data isn’t missing; it’s a labeling mismatch between two valid but different definitions of “giga.”

Converting between the two

Because the prefixes represent different multipliers, converting between, say, GB and GiB requires knowing which definition each figure uses:

• 1 GB (decimal) = 0.9313 GiB
• 1 GiB (binary) = 1.0737 GB
• 1 TB (decimal) = 0.9095 TiB
• 1 TiB (binary) = 1.0995 TB

Use the GB to GiB converter when you need to reconcile a manufacturer’s decimal spec with an operating system’s (mislabeled) binary display.

Where this matters in practice

• RAM is always measured in binary units (even when labeled “GB”) because memory addressing is inherently based on powers of 2 — a “16GB” RAM module genuinely contains 16 × 1,073,741,824 bytes, no discrepancy.
• Storage drives (HDD/SSD) are labeled in decimal units by virtually all manufacturers, following SI convention — the “missing” space when an OS reports a smaller number is a display convention, not a defect.
• Network speeds use decimal units (a “1 Gbps” connection is 1,000,000,000 bits per second) — and note the bits vs. bytes distinction adds a further factor of 8, which is a separate source of confusion (a 1 Gbps connection transfers at most about 125 MB/s, not 1,000 MB/s).
• File sizes reported by operating systems are usually binary math with decimal labels (the exact issue described above), which is why a “4.7 GB” DVD image might show as “4.38 GB” — or vice versa — depending on which tool measured it.

Quick reference table

Decimal	Bytes	Binary equivalent
1 KB	1,000	0.977 KiB
1 MB	1,000,000	0.954 MiB
1 GB	1,000,000,000	0.931 GiB
1 TB	1,000,000,000,000	0.909 TiB

Practical takeaway

When a number “doesn’t add up” between a spec sheet and what your computer displays, the data almost certainly hasn’t gone missing — one of the two figures is using decimal (powers of 1,000) units and the other binary (powers of 1,024), often with both displayed under the same label (“GB”). Use the GB to MB and related converters when you need to move between decimal units, and remember that the ~7% gap between “GB” and “GiB” compounds at larger scales — it’s about 7% at the gigabyte level but closer to 10% at the terabyte level.