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A new 1 TB drive comes out of the box. You plug it in, open the file manager, and it says 931 GB. Within a minute most people are annoyed and a few are typing a complaint. But the drive is full-sized and the computer is not wrong. They simply use two different meanings of the word “giga”. Once you see the two systems side by side, the mystery disappears for good.
Two ways to count a kilobyte
In everyday life “kilo” means 1,000. A kilometre is 1,000 metres, a kilogram is 1,000 grams. Computers, however, work in binary, where the natural round numbers are powers of two. Two to the power of 10 is 1,024, which is very close to 1,000, so early engineers called it a “kilobyte” and moved on. That shortcut was fine when files were small. It became a problem when everything got big.
To end the confusion, the standards bodies defined two separate sets of names. The decimal prefixes (kilo, mega, giga, tera) always mean powers of 1,000. The binary prefixes (kibi, mebi, gibi, tebi) always mean powers of 1,024. A kibibyte is written KiB, a gibibyte GiB. The trouble is that many people, and some software, still say “GB” when they really mean GiB.
| Name | Symbol | Exact size in bytes | Counting system |
|---|---|---|---|
| kilobyte | KB | 1,000 | Decimal (powers of 1,000) |
| kibibyte | KiB | 1,024 | Binary (powers of 1,024) |
| megabyte | MB | 1,000,000 | Decimal |
| mebibyte | MiB | 1,048,576 | Binary |
| gigabyte | GB | 1,000,000,000 | Decimal |
| gibibyte | GiB | 1,073,741,824 | Binary |
| terabyte | TB | 1,000,000,000,000 | Decimal |
| tebibyte | TiB | 1,099,511,627,776 | Binary |
Each binary unit is a little bigger than its decimal twin. The difference is small at KB and large at TB.
- Powers of 1,000
- Matches the metric system
- Used on the box of a drive or SD card
- Powers of 1,024
- Matches how memory chips are built
- Often still labelled “GB” on screen
Neither side is lying. Problems start only when one side’s number is read with the other side’s meaning. Note that macOS and most phones now show decimal sizes, which is why a 1 TB drive often looks like 1 TB there.
Where the 69 GB goes
Take the box that says 1 TB. The maker means 1,000,000,000,000 bytes. Your operating system divides that by 1,073,741,824 (bytes in a GiB) and prints the result with the letters GB. The division gives 931.3. That is the whole story, and no space has been taken away. The same number of bytes is written in a larger unit, so the number gets smaller.
1,000,000,000,000 ÷ 1,073,741,824 = 931.3. The drive still holds exactly the same number of bytes. A little more space is then used by the file system and system files, which is a separate and much smaller effect.
The same arithmetic works for every drive size you can buy. A rule of thumb that is easy to remember: the screen number is about 93% of the box number for anything in gigabytes.
Every pair shows the same 6.9% gap. A 2 TB drive appears to “lose” 137 GB, which sounds dramatic but is the same percentage as the 128 GB drive losing 8.8 GB.
The gap grows as the units get bigger
The ratio between binary and decimal is 1.024 for one step (kilo), 1.024 squared for two steps (mega), and so on. Each extra step multiplies in another 2.4%. That is why nobody noticed the difference in the floppy-disk era and everybody notices it now. Small files hide the gap and big drives expose it.
The gap is 1 − (1000 ÷ 1024)ⁿ. At petabyte scale a “1 PB” system shows up as about 0.89 PiB, so storage vendors and cloud bills must be careful about which unit they mean.
- 1 BByteOne character of plain English text takes about one byte.
- 1 KBKilobyteA short email without pictures is a few kilobytes.
- 1 MBMegabyteA compressed photo from an older phone, or a minute of low-quality music.
- 1 GBGigabyteA short HD video or a few hundred photos.
- 1 TBTerabyteA modern laptop drive. About 1,000 GB.
- 1 PBPetabyteThe scale of big data centres, not laptops.
Bits versus bytes: the lowercase b matters
There is a second mix-up that costs people far more patience than the 931 GB one. A byte is 8 bits. Storage is measured in bytes (capital B): KB, MB, GB. Network speed is measured in bits (lowercase b): Kbps, Mbps, Gbps. When an internet plan says 100 Mbps, it means 100 megabits per second. A file that is 100 MB is 800 megabits.
When a number looks wrong by a factor of eight, check the b. It is almost always bits versus bytes.
- 1Start with the planFor example 100 Mbps, which is 100 megabits a second.
- 2Divide by 8100 ÷ 8 = 12.5 megabytes a second at the very best.
- 3Divide the file by that speedA 1,000 MB file takes 1,000 ÷ 12.5 = 80 seconds.
- 4Expect a little extraWi-Fi, servers and protocol overhead usually leave you with 80 to 95 percent of the plan.
This is also why a speed test result and a download counter never match at first glance. The speed test says 94 Mbps and the browser says 11.7 MB/s. Divide 94 by 8 and you get 11.75. Both are right.
How long will it really take?
Here is a quick reference of the best-case time for files of common sizes. It uses the formula size in MB × 8 ÷ speed in Mbps = seconds, and assumes the full speed is available all the time, which is optimistic.
| File size | 10 Mbps | 50 Mbps | 100 Mbps | 500 Mbps | 1 Gbps |
|---|---|---|---|---|---|
| 100 MB | 80 s | 16 s | 8 s | 1.6 s | 0.8 s |
| 1 GB | 13 min 20 s | 2 min 40 s | 1 min 20 s | 16 s | 8 s |
| 5 GB | 1 h 7 min | 13 min 20 s | 6 min 40 s | 1 min 20 s | 40 s |
| 50 GB | 11 h 7 min | 2 h 13 min | 1 h 7 min | 13 min 20 s | 6 min 40 s |
The ringed cell is a 1 GB file on a 100 Mbps line: 80 seconds. Doubling the speed halves the time, and the darkest cells show why a big game on a slow line becomes an overnight job.
These are rules of thumb for the usual Windows case. Always check which unit your own tool is showing.
Why the sizes on your phone look different
Most phones, macOS and many web services now use decimal units, so a 128 GB phone shows 128 GB of storage, with the operating system taking a chunk. Windows still reports sizes in binary units but labels them with the decimal names. Linux tools often show both, with an option to switch (the “-h” and “--si” flags in common commands). So the same 1 TB drive can honestly look like 1 TB on a Mac and 931 GB on a Windows laptop.
- Windows File Explorer: binary counting, decimal-looking labels. 1 TB shows as about 931 GB.
- macOS and iPhone: decimal counting. A 1 TB drive shows about 1 TB.
- Most Android phones: vary by maker. Check the unit in the storage screen.
- Cloud plans: usually decimal, but read the fine print before you plan around the last few percent.
Practical tips
- Plan with the screen number. If a project needs 900 GB, a 1 TB drive is too tight on Windows. Buy the next size up.
- Check the b. For anything about speed, look for the lowercase b. Divide by 8 for MB per second.
- Keep 10 to 15 percent free. Drives slow down and cannot reorganise files when they are almost full. That slack is far bigger than the unit gap.
- Compress before you upload. Smaller files save time and storage. The image compressor can cut a photo by 80 to 90 percent, and this guide shows how.
- Remember Base64 adds a third. If you embed files as text, the size grows by about 33 percent. See Base64 is not encryption.
If you enjoy the number side of this, the binary, hex and decimal guide explains why 1,024 is such a natural number for computers.
A quick self-test
A 512 GB SD card is full of files. How many GiB will Windows show? Multiply by 1,000,000,000 and divide by 1,073,741,824: about 476.8. And how long does a 2 GB video take to upload on a 20 Mbps upload line? 2,000 MB × 8 ÷ 20 = 800 seconds, about 13 minutes at best. Try your own numbers in the converter above and you will never be surprised by a drive label again.
Questions people ask
Why does my 1 TB drive show only 931 GB?
The maker counts 1 TB as 1,000,000,000,000 bytes. Windows divides by 1,073,741,824 to show gibibytes, but writes the unit as GB. The bytes are all there; the number is just written in a bigger unit.
What is the difference between GB and GiB?
1 GB is 1,000,000,000 bytes (decimal). 1 GiB is 1,073,741,824 bytes (binary, 1,024 cubed). A GiB is about 7.4% larger than a GB.
What is the difference between MB and Mbps?
MB means megabytes, a file size. Mbps means megabits per second, a speed. One byte is eight bits, so 100 Mbps is at best 12.5 MB per second.
How many MB are in a GB?
1,000 MB in a GB by the decimal definition, and 1,024 MiB in a GiB by the binary definition.
Why is RAM sold in binary sizes but drives in decimal?
Memory chips are built from powers of two, so their natural sizes are 4, 8, 16 and 32 GiB. Drive makers adopted the decimal prefixes because they match the metric system and give larger-looking numbers.
Is the missing space a scam?
No. The decimal and binary systems are both documented, and the box states the real byte count. The confusion comes from software showing binary sizes with decimal-looking labels.