A new 1 TB drive plugs in, Windows reports 931 GB, and roughly 69 GB appears to have gone missing. Nothing is missing. The two numbers are the same amount of storage counted two different ways, and this storage unit converter shows both at once so the gap stops being a mystery.
Type any figure into the storage unit converter, pick the unit it came in, and every other unit updates at the same time: bits and bytes, the decimal units a manufacturer prints on the box, and the binary units an operating system counts in. Below the converter is the arithmetic behind the missing 69 GB, a table of the conversions people look up most, and the short list of places where each base is used.
How to Use the Storage Unit Converter
The storage unit converter takes one number and one unit. The amount can be typed with commas or without, and a stray unit typed after the figure is ignored rather than treated as an error. Results appear in every unit at once, and the copy button beside each one puts that value on the clipboard without its thousands separators, ready to paste into a spreadsheet.
The toggle underneath decides what an ambiguous unit means. A figure read off a drive’s packaging is decimal, so 1,000 bytes to the KB is correct for it. A figure read out of This PC or a file’s properties came from Windows, which counts in 1,024 bytes to the KB while still writing GB. Units written KiB, MiB, GiB and TiB are never ambiguous, so the toggle switches itself off when one of those is selected.
1 TB = 931.32 GiB
That is 1,000,000,000,000 bytes, which Windows would display as 931.32 GB.
Enter a number to convert.
Drive manufacturers and macOS count in thousands. Windows counts in 1,024s but still writes GB, which is where the missing space goes.
Bits and bytes
The raw count. Every other unit on this page is a shorthand for one of these two.
- Bits bit
- 8,000,000,000,000
- Bytes B
- 1,000,000,000,000
Decimal units
A thousand to the next unit. What drive manufacturers print on the box.
- Kilobytes KB
- 1,000,000,000
- Megabytes MB
- 1,000,000
- Gigabytes GB
- 1,000
- Terabytes TB
- 1
- Petabytes PB
- 0.001
Binary units
One thousand and twenty four to the next unit. What Windows counts in.
- Kibibytes KiB
- 976,562,500
- Mebibytes MiB
- 953,674.316
- Gibibytes GiB
- 931.322575
- Tebibytes TiB
- 0.909494702
- Pebibytes PiB
- 0.00088817842
The binary units follow the prefixes defined in IEC 60027-2, as published by NIST. Every figure above is converted from the exact byte count, then shown to nine significant figures.
Why a 1 TB Drive Shows 931 GB in Windows
Drive manufacturers count in thousands. One of their gigabytes is 1,000,000,000 bytes, and one terabyte is 1,000,000,000,000 bytes, which Seagate states plainly in its own support documentation. A 1 TB drive really does hold a trillion bytes.
Windows counts in 1,024s. Its gigabyte is 1,073,741,824 bytes, so the same trillion bytes divided by that figure comes to 931.32, and Windows displays 931 GB. Both numbers describe an identical drive. Only the size of the unit changed, in the same way that a distance is a smaller number in miles than in kilometers.
The gap widens with capacity, because the difference compounds at every step of the ladder. A 500 GB drive loses about 34 GB to the conversion, a 1 TB drive about 69 GB, and an 8 TB drive is reported as roughly 7.28 TB. This is also why the shortfall looks worse on large drives and barely noticeable on a small USB stick.
Decimal and Binary Storage Units Explained
The confusion exists because one set of prefixes got used for two different things. Kilo, mega and giga mean 1,000, 1,000,000 and 1,000,000,000 everywhere else in measurement, but early computing borrowed them for the nearest powers of two, which are close but not equal. A second set of prefixes was defined to separate the two, published as IEC 60027-2 and documented by NIST, which is where kibibyte, mebibyte and gibibyte come from.
| Decimal unit | Bytes | Binary unit | Bytes |
|---|---|---|---|
| Kilobyte (KB) | 1,000 | Kibibyte (KiB) | 1,024 |
| Megabyte (MB) | 1,000,000 | Mebibyte (MiB) | 1,048,576 |
| Gigabyte (GB) | 1,000,000,000 | Gibibyte (GiB) | 1,073,741,824 |
| Terabyte (TB) | 1,000,000,000,000 | Tebibyte (TiB) | 1,099,511,627,776 |
The binary names are correct and almost nobody uses them. Windows shows GB when it means GiB, most download pages do the same, and the result is that the label on the screen rarely says which base produced the number underneath it. Working out which one is meant, from where the figure came from, is the skill the units themselves were supposed to remove, and it is why the storage unit converter above shows both families at the same time rather than making that choice for the reader.
Common Storage Conversions at a Glance
These are the conversions people look up most: the capacity printed on a drive against the figure Windows reports for it. Every row is the decimal capacity converted to its binary equivalent, rounded to two decimal places, and the storage unit converter above will produce the same figures for any capacity not listed here.
| Advertised capacity | Reported by Windows | Exact bytes |
|---|---|---|
| 128 GB | 119.21 GB | 128,000,000,000 |
| 256 GB | 238.42 GB | 256,000,000,000 |
| 500 GB | 465.66 GB | 500,000,000,000 |
| 512 GB | 476.84 GB | 512,000,000,000 |
| 1 TB | 931.32 GB | 1,000,000,000,000 |
| 2 TB | 1.82 TB | 2,000,000,000,000 |
| 4 TB | 3.64 TB | 4,000,000,000,000 |
| 8 TB | 7.28 TB | 8,000,000,000,000 |
| 16 TB | 14.55 TB | 16,000,000,000,000 |
Windows truncates the last digit in places rather than rounding it, so a 2 TB drive can read 1.81 TB where the arithmetic above gives 1.82 TB. Seagate’s own capacity table lists the truncated figures for that reason. Both describe the same drive, and a digit of disagreement in the last place is normal.
A quick sanity check without any tool at all: multiply a decimal terabyte figure by 0.909 to get the terabytes Windows will show, or by 931 to get the gigabytes. That is close enough to tell a normal conversion apart from a genuine fault.
Where Each Base Is Used
Knowing which base produced a number is most of the battle, and the pattern is consistent enough to memorize. Running a figure through the storage unit converter in the wrong base produces an answer that is off by about 2.4% at every step of the ladder, which compounds to 7.4% by the time it reaches gigabytes and is exactly the size of the discrepancy people come looking to explain.
- Drive and USB manufacturers use decimal. The capacity on the box, on the label and in the product listing is counted in thousands.
- Windows uses binary and writes decimal names. Anything reported by File Explorer, This PC or a file’s properties dialog is a binary figure labeled KB, MB or GB.
- macOS reports the decimal figure, which is why the same external drive can read 1 TB on a Mac and 931 GB on a PC without either being wrong.
- Memory is always binary. A 16 GB memory module holds 17,179,869,184 bytes, because memory is addressed in powers of two and cannot be built to a round decimal size.
- Network speeds are counted in bits, not bytes. A 100 Mbps connection moves at most 12.5 megabytes a second, so a figure taken from a speed test has to be divided by eight before it means anything about file sizes.
Phone and tablet storage follows the drive makers, so a 128 GB phone has 128 billion bytes on paper and shows something closer to 119 GB before the operating system takes its share.
What a Storage Unit Converter Cannot Tell You
Unit conversion explains the difference between the number on the box and the number in the drive’s properties. It does not explain the smaller gap between that figure and the free space actually available, because a storage unit converter only handles arithmetic and the rest of the shortfall is real space genuinely in use.
- The file system takes a share. Formatting a drive builds the structures that track where everything is stored, and those live on the drive itself.
- Prebuilt machines ship with hidden partitions. A recovery partition of several gigabytes is normal and does not appear as usable space.
- SSDs hold spare capacity in reserve. Drives keep a portion back to replace worn cells and to keep write performance up, and that reserve is never visible to the operating system.
Those are all measured in gigabytes rather than tens of gigabytes, so they explain a small shortfall and never a large one. To see the raw figure before any of it is deducted, open Disk Management, where the drive’s total size and each partition on it are listed separately.