Data Storage Converter: GB, MB, GiB and TB
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Data Storage Converter
Bytes, kilobytes, megabytes, gigabytes and terabytes, with the decimal and binary versions of each kept separate. That separation is the whole point: it is why a drive sold as 1 TB shows up as 931 GB once you plug it in.
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Advertised size against what the operating system shows
| Sold as | Bytes | Shown as | Shortfall |
|---|
Every row and chip is a live link you can bookmark or share.
How to use this tool
Pick an amount and two units. The unit list deliberately spells out which system each one belongs to, so kilobytes and kibibytes sit next to each other rather than hiding behind the same abbreviation. If you are not sure which you want, the decimal ones are what manufacturers and network operators use and the binary ones are what your operating system reports.
The red box underneath translates the result into the question people actually have, which is why a drive never shows the size on the box. It gives the byte count, both readings of it, and the percentage gap.
Why a 1 TB drive shows up as 931 GB
Nothing is missing from the drive, and nobody is being cheated. Two different definitions of the same word are in use at once.
Storage manufacturers use powers of ten, the same way every other SI prefix works. A kilometre is a thousand metres, so a kilobyte is a thousand bytes and a terabyte is a million million. By that count a 1 TB drive holds exactly 1,000,000,000,000 bytes, and it does.
Computers address memory in powers of two, so early programmers borrowed the prefixes and used them for the nearest binary round number. A kilobyte became 1024 bytes, because 2 to the tenth is 1024 and that is close enough to a thousand to reuse the word. Windows still divides by 1024 at every step and still writes the answer as GB.
The gap compounds. It is 2.4 percent at kilobytes, 4.9 at megabytes, 7.4 at gigabytes and 10.0 at terabytes. Divide a trillion bytes by 1073741824 and you get 931.32, which is the number in your file manager. The correct name for that unit is the gibibyte, and the standard defining it has existed since 1998, but almost nothing in consumer software uses the name.
Two exceptions are worth knowing. Apple switched macOS to decimal units in 2009, so a Mac reports the same 1 TB the box does. And memory is genuinely binary: a 16 GB stick of RAM really is 16 times 1073741824 bytes, because memory chips are addressed in powers of two in a way that spinning platters and flash blocks are not.
A worked check you can repeat on paper
Start with a 1 TB drive. That is 1,000,000,000,000 bytes by the manufacturer definition. Divide by 1024 for kibibytes, again for mebibytes, and again for gibibytes: that is dividing by 1,073,741,824 in total. The answer is 931.3226 GiB, which your computer will display as 931 GB.
Check the percentage. 931.3226 divided by 1000 is 0.9313, so you appear to lose 6.87 percent going from TB to the GiB reading. Going from TB to TiB directly, 1 divided by 1.099511627776 is 0.909495, a 9.05 percent gap. Both are correct, they are simply different comparisons, which is exactly why this confuses people.
One more that catches people. Network speeds are quoted in bits, not bytes, and there are eight bits to a byte. A 100 megabit connection moves 12.5 megabytes per second at best, so a 1 GB file takes about 80 seconds rather than 10.
Mistakes that catch people out
Confusing megabits with megabytes. A capital B means bytes and a lowercase b means bits. Internet plans are sold in bits because the number is eight times larger and looks better.
Assuming the shortfall is the same at every size. It is not. A 128 GB phone loses about 7.4 percent to the binary reading, a 16 TB drive loses 9.05 percent. The bigger the unit, the wider the gap.
Comparing a Mac and a PC and concluding one is wrong. Since 2009 macOS reports decimal units and Windows reports binary ones. The same drive will honestly show two different numbers on the two machines.
Forgetting formatting and reserved space. The binary and decimal difference explains most of the missing capacity but not all of it. A filesystem also reserves space for its own bookkeeping, and on a phone the operating system itself takes a share before you see anything.
Popular storage conversions
The conversions people look up most. Each opens the tool ready to go with its own address.
How this was checked
Every factor is an exact integer, so there is nothing to approximate. A gibibyte is 1073741824 bytes and a gigabyte is 1000000000, both confirmed against their definitions rather than typed from memory. The derived figures were then checked: 1 TB comes to 931.32 GiB, and the TB against TiB gap comes to 9.05 percent.
The compounding pattern was verified step by step, giving 2.4, 4.9, 7.4 and 10.0 percent at kilo, mega, giga and tera. Those numbers are not estimates, they follow from 1024 divided by 1000 raised to the power of the step.
The bits to bytes relationship was checked separately, along with the download time example: a gigabyte is eight gigabits, so at 100 megabits per second it takes 80 seconds.
Related Calculators and Reference
See also the Binary, Hex and Decimal Converter for the powers of two underneath every storage unit on this page.
- Scientific and Engineering Notation Converter for the SI prefixes that storage units borrowed and then redefined.
- Percentage Calculator for working out the shortfall on any size yourself.
- Roman Numeral Converter for another number system that does not use place value.
- Volume and Capacity Converter for the same problem in a different field, where one word means two different sizes.
- Mbps to MB/s Converter: internet speeds and download times.
- NIST on binary and decimal prefixes explains why the kibibyte exists and when it was standardised.