GB, MB and Why Your 1 TB Drive Shows 931 GB

By Hutu Laurentiu

5 min read · Updated October 2026

You bought a "1 TB" drive and the computer says 931 GB. Nobody stole 69 gigabytes — you've just met the two incompatible definitions of a gigabyte that have coexisted for sixty years.

Two systems, two truths

• Decimal (SI): kilo=1000, mega=1000², giga=1000³. Drive manufacturers use this. It matches every other unit in physics.
• Binary (IEC): kibi=1024, mebi=1024², gibi=1024³. Computers address memory in powers of two, so operating systems traditionally count this way.

A 1,000,000,000,000-byte drive is exactly 1 TB (decimal) but also 909.5 GiB (binary). Windows labels those GiB as "GB" — hence "931 GB". macOS switched to decimal counting years ago; Linux shows whichever you ask it for. The bytes are identical; only the ruler changed.

The quick conversion rules

• Binary ladder: 1 GiB = 1024 MiB, 1 MiB = 1024 KiB, 1 KiB = 1024 bytes.
• Decimal ladder: 1 GB = 1000 MB = 1,000,000,000 bytes.
• Cross-system: 1 GB = 0.931 GiB. 1 GiB = 1.074 GB. The gap grows by 2.4% per rung.

So: 500 GB drive = 465.7 GiB ("465 GB" in Windows). 128 GB phone = 119.2 GiB. 1 TB = 909.5 GiB. The multiplier 0.931 works for any TB→"Windows GB" conversion.

Practical storage planning

When a spec says your camera "records 400 MB/min", that's almost certainly decimal MB. To plan how many minutes fit on a 256 GB card: 256×0.931 = 238 GiB usable-ish (before formatting overhead, ~2-4% more lost), then 238×1024 ÷ 400 ≈ 609 minutes. Always chain: advertised → decimal → binary → minus overhead. Beginners forget one link and overbuy.

Why RAM never converted

Memory modules are sold in powers of two by construction — 8 GiB RAM is physically 8×1024³ bytes because that's how address lines work. So RAM has no "lost" capacity; drives and SSDs (marketed in decimal) do. That's also why "16 GB RAM + 512 GB storage" mixes two counting systems in one sentence and both are correct.

Internet speeds: the third wrinkle

Connection speeds use bits, not bytes: 300 Mbps = 37.5 MB/s (divide by 8, then by 1000). Downloads of 1 GB take ~27 s on that line — plus protocol overhead, so expect ~30. Mixing bits and bytes is the classic beginner's error when "why is my download slow?" questions appear online.