You buy a 1TB drive, plug it in, and Windows reports 931GB. Roughly 69GB appears to have vanished before you have stored anything. Nothing is faulty and nobody is cheating — two different definitions of the word "gigabyte" have been in simultaneous use for fifty years. This page gives the exact arithmetic, then accounts for the space that genuinely does disappear.
The arithmetic
The drive holds 1,000,000,000,000 bytes, exactly as advertised. Windows divides by 1024 three times to reach "GB": 1,000,000,000,000 ÷ 1024 ÷ 1024 ÷ 1024 = 931.32. Same bytes, different definition of the same word. To predict any drive's reported size, multiply the advertised figure by 0.9313.
Two definitions, one word
The SI prefixes are unambiguous and have been since 1795: kilo means one thousand, mega one million, giga one billion. They are powers of ten because the metric system is decimal.
Computers address memory in powers of two, and early programmers noticed that 210 = 1024 is conveniently near 1000. Rather than coin new words, they borrowed the metric prefixes. A "kilobyte" became 1024 bytes.
At the kilobyte scale the discrepancy is 2.4% — small enough to ignore. But the error compounds with each step:
| Unit | Decimal (SI) | Binary (IEC) | Difference |
|---|---|---|---|
| Kilo | 1,000 | 1,024 | 2.4% |
| Mega | 1,000,000 | 1,048,576 | 4.9% |
| Giga | 1,000,000,000 | 1,073,741,824 | 7.4% |
| Tera | 1012 | 1,099,511,627,776 | 10.0% |
| Peta | 1015 | 250 | 12.6% |
At terabyte scale the gap is 10%, which is large enough to look like something is wrong. And it grows: at petabyte scale it is 12.6%.
The IEC prefixes
In 1998 the International Electrotechnical Commission standardised separate binary prefixes to end the ambiguity:
| Name | Symbol | Bytes | Power |
|---|---|---|---|
| Kibibyte | KiB | 1,024 | 210 |
| Mebibyte | MiB | 1,048,576 | 220 |
| Gibibyte | GiB | 1,073,741,824 | 230 |
| Tebibyte | TiB | 1,099,511,627,776 | 240 |
These are correct, unambiguous, and largely ignored — mainly because the names sound faintly ridiculous. Linux adopted them properly. macOS solved the problem differently in 2009 by switching Finder to decimal units, so a Mac reports a 1TB drive as roughly 1TB. Windows continues to compute binary values and label them with decimal prefixes, which is precisely the combination that generates the confusion.
💡 Why manufacturers are on solid ground
The SI definitions predate computing by 150 years. A manufacturer stating 1TB = 1012 bytes is using the word correctly by every international standard. Class-action lawsuits over this have generally been resolved in manufacturers' favour, with the outcome being clearer labelling rather than changed capacities. The unusual definition is the computing one.
What each drive size should report
| Advertised | Actual bytes | Windows shows | Formatted, roughly |
|---|---|---|---|
| 16 GB | 16,000,000,000 | 14.9 GB | ~14.6 GB |
| 32 GB | 32,000,000,000 | 29.8 GB | ~29.2 GB |
| 64 GB | 64,000,000,000 | 59.6 GB | ~58.4 GB |
| 128 GB | 128,000,000,000 | 119.2 GB | ~117 GB |
| 256 GB | 256,000,000,000 | 238.4 GB | ~234 GB |
| 500 GB | 500,000,000,000 | 465.7 GB | ~458 GB |
| 512 GB | 512,000,000,000 | 476.8 GB | ~468 GB |
| 1 TB | 1012 | 931.3 GB | ~915 GB |
| 2 TB | 2 × 1012 | 1862.6 GB | ~1831 GB |
| 4 TB | 4 × 1012 | 3725.3 GB | ~3662 GB |
| 8 TB | 8 × 1012 | 7450.6 GB | ~7324 GB |
If your drive reports close to the third column, everything is normal. If it reports substantially less than the fourth, something else is going on — read on.
Space the filesystem genuinely takes
Beyond the unit conversion, a formatted drive really does lose capacity. A filesystem has to store its own bookkeeping somewhere.
| Filesystem | Overhead | What it stores |
|---|---|---|
| NTFS | ~1–2%, plus MFT reservation | Master File Table, journal, bitmaps, security descriptors |
| exFAT | <1% | Allocation table and bitmap — deliberately minimal |
| FAT32 | <1% | Two file allocation tables |
| APFS | ~1% | Checkpoints, container metadata |
| ext4 | ~1.5%, plus 5% reserved | Inode tables, journal, superblock copies |
Two of these deserve explanation.
NTFS reserves about 12.5% of the volume as a contiguous zone for Master File Table growth. This is not lost — it shrinks automatically as the disk fills — but it can make free space appear lower than expected on a nearly empty drive.
ext4 reserves 5% for root by default. The purpose is genuinely useful: if a runaway process fills the disk, a normal user cannot consume that last 5%, so root can still log in and fix things. On a data-only drive with no system files it is wasted, and can be reduced:
Cluster size and slack space
Filesystems allocate space in fixed blocks called clusters — typically 4KB. A file always occupies a whole number of clusters, so a 1-byte file consumes 4KB and a 5KB file consumes 8KB. The unused remainder is slack space.
For most drives this is negligible. For one holding hundreds of thousands of tiny files — a source code repository, a mail store, a cache — it adds up to gigabytes, and explains why "size" and "size on disk" diverge so sharply in a folder's properties.
Hidden partitions
If the arithmetic still does not account for your missing space, look for partitions Explorer never shows.
| Partition | Typical size | Purpose |
|---|---|---|
| EFI System Partition | 100–500 MB | Boot loader |
| Microsoft Reserved | 16–128 MB | Partitioning metadata |
| Windows Recovery | 500 MB – 1 GB | Repair environment |
| OEM Recovery image | 10–25 GB | Factory restore |
| macOS Recovery | ~1 GB | Recovery mode |
| Swap or hibernation | Varies with RAM | Virtual memory |
The OEM recovery image is the big one — on a laptop it can occupy 25GB, which on a 256GB drive is a tenth of the total. It can usually be removed once you have created recovery media, though doing so forfeits one-key factory restore.
SSD over-provisioning
SSDs contain more flash than they advertise. A "1TB" SSD typically holds around 1024 GiB of physical NAND while presenting 1000 GB to the operating system — the difference is over-provisioning, and it is not a loss.
The controller uses that hidden capacity for wear levelling, garbage collection, and replacing cells as they fail. It is why an SSD maintains write performance when nearly full and why it can keep working as individual cells wear out. Some enterprise drives over-provision by 28% or more, deliberately trading capacity for endurance and consistent latency.
✅ Leave 10–20% free on any SSD
Beyond the manufacturer's over-provisioning, keeping some of the visible capacity free gives the controller more room to work. A completely full SSD must erase and rewrite entire blocks to make room for small writes — a process called write amplification — and performance can drop dramatically. This applies to spinning drives too, where fragmentation increases sharply above about 85% full.
Other space consumers on Windows
| Consumer | Typical size | Manage with |
|---|---|---|
| Hibernation file | Equal to installed RAM | powercfg /hibernate off |
| Page file | 1–2× RAM | System → Advanced → Virtual memory |
| System Restore | Up to 10% of the volume | System Protection settings |
| Windows.old | 10–30 GB | Disk Cleanup — removed automatically after 10 days |
| Component store (WinSxS) | 5–10 GB | DISM /Cleanup-Image /StartComponentCleanup |
| Delivery Optimisation cache | Several GB | Storage settings |
On a machine with 32GB of RAM, hibernation and the page file alone can occupy 60GB before a single document is saved. Disabling hibernation on a desktop that never hibernates recovers the full amount immediately.
Working out where it went
- Multiply the advertised size by 0.9313. That is what Windows should report.
- Subtract 1–3% for the filesystem. That is the formatted figure.
- Open Disk Management and look for partitions without drive letters.
- Check hibernation and page file sizes, which scale with installed RAM.
- Run a disk usage analyser such as WizTree or TreeSize to see actual file consumption.
- Compare size against size on disk for folders full of small files.
If the total still does not reconcile after all that, the drive may genuinely be faulty or — for a cheap USB stick from an unfamiliar seller — deliberately misreporting its capacity. Counterfeit flash drives are firmware-modified to claim a large size while containing far less, and they appear to work until the real capacity is exceeded, at which point data written earlier is silently overwritten. Utilities such as f3 or H2testw verify true capacity by writing and reading back the entire drive.
Verifying a file transferred intact?
Generate a checksum before and after copying to confirm nothing was corrupted — runs in your browser, so it works on files of any size or sensitivity.
Open the Hash Generator →Summary
- 1TB = 1,000,000,000,000 bytes = 931.32 GiB. No space is missing.
- Multiply by 0.9313 to predict what Windows will report.
- Manufacturers use the standard SI definitions. Computing's use of the prefixes is the unusual one.
- Windows labels binary values with decimal prefixes, which is what causes the confusion. macOS uses decimal throughout.
- Formatting costs another 1–3%, and NTFS reserves space for MFT growth that later shrinks.
- Hidden recovery partitions can occupy 25GB on a laptop.
- SSD over-provisioning is invisible and beneficial — leave 10–20% free as well.
- Verify cheap USB drives. Counterfeits report a false capacity and destroy data silently.
Frequently Asked Questions
Why does a 1TB drive show as 931GB?
Manufacturers count a terabyte as 1,000,000,000,000 bytes, while Windows divides by 1024 at each step and labels the result GB. One trillion bytes divided by 1024 three times is 931.32 — so Windows displays 931GB. No space is missing; the two are measuring the same bytes with different definitions of the same word.
Is the manufacturer cheating?
No. The SI prefixes have meant powers of ten since 1795 — kilo means exactly one thousand. Manufacturers use the standard definition. Computing borrowed the same prefixes for powers of two because 1024 is conveniently close to 1000, and the ambiguity has persisted ever since. Courts have generally sided with manufacturers on this.
What is the difference between GB and GiB?
A gigabyte (GB) is 1,000,000,000 bytes. A gibibyte (GiB) is 1,073,741,824 bytes — 2 to the power of 30. The binary prefixes were standardised in 1998 specifically to remove the ambiguity. macOS and Linux report in true GB; Windows reports GiB values while labelling them GB, which is where the confusion comes from.
Why do I lose more space after formatting?
The filesystem needs room for its own structures — the file table, journal, allocation bitmaps and boot sectors. NTFS also reserves around 12.5% of the volume for Master File Table growth, though that reservation shrinks automatically as the disk fills. Typical formatting overhead is 1 to 3% beyond the unit conversion.
Where has my SSD capacity gone if the numbers still do not add up?
Check for hidden partitions. Most laptops ship with an EFI system partition, a recovery partition, and often a manufacturer restore image, which together can occupy 20 to 30GB and do not appear in Explorer. Open Disk Management to see every partition on the drive, including the ones without a drive letter.