The moment everyone notices it
You buy a drive advertised as 1TB, plug it in, and your operating system reports something like 931GB of usable space. Cue the immediate suspicion: did the manufacturer shortchange you? Is some of the space reserved for hidden system files? The real answer is simpler and much less sinister — your drive almost certainly does contain the full amount of storage advertised. The "missing" space is an illusion created by two different, both technically valid, definitions of what a "gigabyte" means.
Two different gigabytes
The confusion comes down to powers of 1000 versus powers of 1024.
Decimal (SI) units — the ones drive manufacturers use on the box — treat kilo, mega, giga, and tera as clean powers of 1000, exactly like every other measurement in the metric system: 1 KB = 1,000 bytes, 1 MB = 1,000,000 bytes, 1 GB = 1,000,000,000 bytes, and 1 TB = 1,000,000,000,000 bytes. This matches how every other unit prefix works (a kilometer is 1,000 meters, not 1,024), and it makes for clean, round marketing numbers.
Binary units — the ones your operating system has historically used internally, often while still (confusingly) labeling them "GB" or "TB" — treat the same prefixes as powers of 1024, because computers fundamentally work in binary and 1024 (2^10) is the power-of-two value closest to 1000: 1 KiB = 1,024 bytes, 1 MiB = 1,024² bytes, 1 GiB = 1,024³ bytes, and 1 TiB = 1,024⁴ bytes. The correct symbols for these binary units are KiB, MiB, GiB, and TiB (the "i" stands for "binary") — but many operating systems still display them with the plain "GB"/"TB" labels borrowed from the decimal system, which is the entire root of the confusion.
Doing the actual math
A drive that is genuinely, fully 1,000,000,000,000 bytes (1 TB, decimal, as advertised on the box) works out to 1,000,000,000,000 ÷ 1,024³ ≈ 931.32 when expressed in the binary unit your OS is actually using internally — which it then mislabels "GB" instead of the correct "GiB". No bytes went missing. No hidden partition ate your space. It's the same physical storage, described with two different, equally legitimate but incompatible measuring sticks — and the gap grows larger the bigger the drive gets, which is exactly why it's barely noticeable on a small USB stick but glaring on a multi-terabyte drive.
Why this still hasn't been "fixed"
The IEC standardized the KiB/MiB/GiB naming convention specifically to resolve this ambiguity back in the late 1990s, and some operating systems and file managers have since adopted it correctly. But decades of software, documentation, and user habit built around the sloppy "GB means whichever one my OS feels like today" convention means the confusing mislabeling persists in plenty of places, and probably will for a long while yet.
How to avoid getting confused by it
When you see a storage number, ask which convention is being used: marketing material and drive packaging almost always mean decimal (1000-based); your operating system's "properties" or "disk usage" panel is very often actually binary (1024-based) math wearing a decimal label. When precision matters — comparing drive capacities, planning storage budgets, or explaining the discrepancy to someone else — converting the same value into both scales side by side removes all ambiguity at a glance.
Bottom line
Your 1TB drive isn't lying to you and it isn't missing storage — it's just being measured with two different, equally valid rulers that happen to disagree by about 7% at the gigabyte scale and more at larger scales. Once you know both numbers, the "missing" space stops being mysterious and becomes just arithmetic.
Frequently Asked Questions
Yes — File Size Unit Converter converts any value across bytes, KB/KiB, MB/MiB, GB/GiB, TB/TiB, and PB/PiB instantly, and shows both decimal and binary results side by side.
Because drive manufacturers use decimal units (1 TB = 1,000,000,000,000 bytes) while your operating system typically calculates and displays free space in binary units (1 TiB = 1,024^4 bytes) while still labeling it "GB" or "TB" — this tool's explainer breaks down the exact math.
Yes — enter a value in any decimal or binary unit and the tool converts it to every other unit in both systems simultaneously, so you can go from GB to GiB or GiB to GB with equal ease.
It's a one-time $3.99 purchase — no subscription, no account, and no recurring charge. Open it whenever you need a quick conversion.
KB (kilobyte) is the decimal unit equal to 1,000 bytes, while KiB (kibibyte) is the binary unit equal to 1,024 bytes — they represent slightly different amounts of data despite sounding similar, which is exactly the confusion this tool is built to clear up.