Why a 1 TB Drive Shows 931 GB: Decimal vs Binary Units
Buy a 1 TB drive, plug it in, and your computer may report about 931 GB. No space has been taken from you. Two different definitions of 'giga' and 'tera' are in use, and the label on the box and the operating system sometimes use different ones.
This article explains the two systems, shows the arithmetic behind the missing gigabytes, and gives a table so you can predict what an advertised capacity will look like on screen.
Two ways to count: 1,000 or 1,024
Storage makers and the metric system use decimal prefixes: 1 kilobyte is 1,000 bytes, 1 megabyte is 1,000,000 bytes, and so on. Computer memory, however, is built in powers of two, so for decades the same words were used for 1,024-based amounts: 1 KB was 1,024 bytes (2^10), 1 MB was 1,048,576 bytes (2^20) and 1 GB was 1,073,741,824 bytes (2^30).
To remove the ambiguity, the International Electrotechnical Commission defined separate binary names: kibibyte (KiB), mebibyte (MiB), gibibyte (GiB) and tebibyte (TiB). The table shows both families side by side.
| Decimal unit | Bytes | Binary unit | Bytes |
|---|---|---|---|
| 1 kB | 1,000 | 1 KiB | 1,024 |
| 1 MB | 1,000,000 | 1 MiB | 1,048,576 |
| 1 GB | 1,000,000,000 | 1 GiB | 1,073,741,824 |
| 1 TB | 1,000,000,000,000 | 1 TiB | 1,099,511,627,776 |
The binary system exists for a practical reason. Memory chips are addressed with binary signals, so a block with 10 address lines holds exactly 2^10 = 1,024 locations. Counting in 1,024s matched the hardware, and 1,024 is close enough to 1,000 that the shared names stuck.
The confusion starts when software calculates in binary units but prints the decimal symbol. Windows has historically shown binary amounts labelled GB, while recent versions of macOS report decimal amounts. The same drive can therefore look different on two computers.
Worked example: where the 931 GB comes from
capacity in GiB = bytes ÷ 1,073,741,8241,073,741,824 is 1,024 × 1,024 × 1,024.
A drive sold as 1 TB holds 1,000,000,000,000 bytes. Dividing by 1,073,741,824 gives 931.3 GiB. In tebibytes, 1,000,000,000,000 ÷ 1,099,511,627,776 = 0.909 TiB. The same number of bytes is simply being expressed in a larger unit, so the figure on screen is smaller.
The gap widens with each prefix, because the 2.4% difference at the kilo level compounds: a binary gigabyte is 1.024 × 1.024 × 1.024 times the decimal one. The number shown is smaller by about 2.3% at the kilo level, 4.6% at mega, 6.9% at giga and 9.1% at tera.
| Advertised capacity | Shown in GiB |
|---|---|
| 128 GB | 119.2 |
| 256 GB | 238.4 |
| 512 GB | 476.8 |
| 1 TB (1,000 GB) | 931.3 |
| 2 TB (2,000 GB) | 1,862.6 |
Second example, will it fit? A folder shows 450 GB in an operating system that counts in binary units, and you want to copy it to a drive sold as 500 GB. In real bytes the folder is 450 × 1,073,741,824 = 483.2 billion bytes, which is 483.2 decimal gigabytes. The drive holds 500 billion bytes, so the folder fits with about 16.8 GB to spare, before allowing for the file system's own overhead. Comparing the raw labels (450 against 500) would make the margin look three times bigger than it is.
On top of the unit difference, the file system, system files and any reserved areas take some space, so the usable amount is a little lower still.
Memory, networks and everything else
Random access memory is built in binary sizes, so '16 GB' of RAM is normally 16 GiB, which is 16 × 1,073,741,824 = 17,179,869,184 bytes. If you read that as a decimal 16 GB you would be about 7% short.
Network speeds always use decimal prefixes: 1 Gbps is exactly 1,000 Mbps. Combining a binary file size with a decimal speed gives estimates that are slightly off, which is rarely important for planning a download. The article on Mbps vs MB/s covers that side of the problem.
Comparing capacities without confusion
- Check whether the number you are reading uses GB or GiB, and whether the software says which.
- If in doubt, convert each figure to bytes. Decimal gigabytes multiply by 1,000,000,000 and binary gibibytes by 1,073,741,824.
- Compare the byte counts, or convert both to a single unit before deciding whether a file fits.
The data units converter handles bits, bytes and every prefix up to petabytes, and the GB to TB and MB to GB converters cover the most common single steps. Both conventions are in use, so check which one a given converter applies before comparing with a figure from your operating system.
- Data Units Converter: Pick a number, a unit to convert from and a unit to convert to. Storage sizes convert to storage sizes and transfer speeds to transfer speeds; tick the SI box to count in thousands instead of 1,024s.
- GB to TB Converter: Enter a size in gigabytes and see it in terabytes. The converter divides by 1,024 unless you tick the SI box, which divides by 1,000.
- MB to GB Converter: Type a size in megabytes and see it in gigabytes at once, rounded to four decimals. Tick the SI option when you want 1,000 MB to the gigabyte.
This article is general information, not professional advice. See our Disclaimer and Editorial Policy.
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