GB to GiB
Convert gigabytes to gibibytes. Divide by 1.073741824, and see why a 1 TB drive reports as 931 GB in Windows.
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The formula
GiB = GB × 1,000,000,000 ÷ 1,073,741,824
= GB ÷ 1.073741824A gigabyte is 10⁹ bytes. A gibibyte is 2³⁰ bytes, which is 1,073,741,824. The ratio is exact and the difference is 7.4%.
This is the page for “why is my 1 TB drive only 931 GB”
Nothing is missing from the drive. Two unit systems are being used at once, and one of them is using the other’s names.
- The manufacturer counts in decimal: a 1 TB drive holds exactly 1,000,000,000,000 bytes. This is correct and it is what SI prefixes mean.
- Windows counts in binary — dividing by 1024 three or four times — but prints the labels “GB” and “TB” rather than “GiB” and “TiB”.
So the same drive is described twice, accurately by the box and misleadingly by the operating system:
| Advertised | Bytes | Windows reports |
|---|---|---|
| 128 GB | 128,000,000,000 | 119 GB |
| 256 GB | 256,000,000,000 | 238 GB |
| 500 GB | 500,000,000,000 | 465 GB |
| 512 GB | 512,000,000,000 | 476 GB |
| 1 TB | 1,000,000,000,000 | 931 GB |
| 2 TB | 2,000,000,000,000 | 1.81 TB |
| 4 TB | 4,000,000,000,000 | 3.63 TB |
| 8 TB | 8,000,000,000,000 | 7.27 TB |
macOS has reported decimal gigabytes since Snow Leopard in 2009, so the same drive plugged
into a Mac shows the number on the box. Linux varies by tool: df -h gives binary by
default, df -H gives decimal.
Quick reference
| GB | GiB |
|---|---|
| 1 GB | 0.931 GiB |
| 1.074 GB | 1 GiB |
| 10 GB | 9.313 GiB |
| 100 GB | 93.132 GiB |
| 500 GB | 465.661 GiB |
| 1000 GB | 931.323 GiB |
Where the drive really does lose space
Separately from the unit confusion, a formatted drive genuinely holds less than its raw capacity:
- Filesystem overhead — the allocation tables, journal and metadata take a fraction of a percent to a couple of percent depending on the filesystem.
- Recovery and vendor partitions on a pre-built machine, which can be tens of gigabytes.
- Over-provisioning on SSDs — spare blocks reserved for wear levelling, invisible to the operating system and not counted in the advertised capacity at all.
A 1 TB SSD showing 915 GB free on a new laptop is 931 GiB of capacity minus a recovery partition and a filesystem. None of that is the 7.4%.
RAM is the other way round
Memory is genuinely binary: an “8 GB” DIMM is 8 GiB, or 8,589,934,592 bytes. So the industry uses decimal prefixes for storage and binary ones for memory, with the same names for both — which is the whole reason the IEC introduced KiB, MiB and GiB in 1998.
Estimating in your head
Take off 7%. 1000 GB minus 70 is 930 GiB, against a true 931.3. For terabytes, take off 10%: a 4 TB drive shows about 3.6 TiB.