What this tool does
This converter moves between bytes, kilobytes, megabytes, gigabytes and terabytes in either SI (decimal) or binary (IEC) notation. Pick your from-unit, your to-unit, choose the unit system, and the result updates instantly in your browser.
The reason this needs a separate mode toggle at all: "kilobyte" has meant two different quantities for decades, and neither meaning has won. Pick the wrong one and a file size, a download estimate, or a drive capacity comes out wrong by a growing margin the larger the number gets.
The KB/KiB mess
SI (International System of Units) prefixes — kilo, mega, giga, tera — have always meant powers of ten, the same as a kilogram is 1,000 g. Computing borrowed the prefixes but, because memory is addressed in binary, quietly redefined them as powers of two: a "kilobyte" of RAM was 1,024 bytes, not 1,000. Two incompatible meanings shared one label for decades. The IEC settled the ambiguity in 1998 by minting new names for the binary values — kibibyte (KiB), mebibyte (MiB), gibibyte (GiB), tebibyte (TiB) — leaving KB/MB/GB/TB free to mean what SI always said they meant. Adoption of the new names has been slow and partial, which is the actual source of the ongoing confusion.
Storage marketing vs OS reporting
Drive and flash manufacturers price and label capacity in SI units — a "1 TB" SSD means 1,000,000,000,000 bytes, calculated the decimal way, because it produces the larger, more marketable number for a given amount of silicon. Operating systems, meanwhile, largely inherited the old binary convention for file and drive sizes: Windows File Explorer divides by 1,024 at each step but still prints the label "GB", not "GiB". Buy that 1 TB drive and Windows reports it as roughly 931 GB — not because storage is missing, but because the OS is counting in gibibytes while wearing a gigabyte's name tag. macOS switched its Finder to genuine SI units in Snow Leopard (10.6, 2009), so the same physical drive is reported as a different number depending on which OS is doing the counting, even though not a byte has moved.
Worked example
Take 1 GB. In SI mode:
1 GB × 1,000 = 1,000 MB
In binary mode the same starting value scales by 1,024 instead of 1,000 at each step, so 1 GiB = 1,024 MiB = 1,073,741,824 bytes — a larger byte count for the "same" 1-unit figure than the SI path gives. Toggle the mode on the tool above to see both readings for any value.
When the gap starts to matter
At small sizes the SI/binary gap is easy to ignore: a "1 KB" file is either 1,000 or 1,024 bytes, a 2.4% difference nobody notices. But the gap compounds with every prefix step, because it's applied again at each power. By megabytes it's roughly 4.9%; by gigabytes roughly 7.4%; by terabytes it reaches about 10% (1 TB = 1,000,000,000,000 bytes vs 1 TiB = 1,099,511,627,776 bytes, a 9.95% difference). That's the scale at which "where did my space go" complaints cluster — a 4 TB drive genuinely reads as roughly 3.6 TB in a binary-reporting OS, before accounting for the file system's own overhead or a recovery partition.
Common uses
- Storage shopping: checking what a drive's SI-labelled capacity will actually show once your OS reports it in binary.
- Network speeds: ISPs quote speeds in Mbps (megabits per second, SI, and note — bits, not bytes). To estimate how long a 4 GB file takes to download, convert the file size to megabits and divide by your speed in Mbps.
- Cloud and container limits: Docker memory limits, AWS instance storage, and database size quotas may quietly use either convention depending on the provider.
- File-transfer estimates: converting a folder size in GiB to bytes to check it will actually fit on removable media sized in GB.
Frequently asked
Is a gigabyte 1,000 MB or 1,024 MB?
In SI it is exactly 1,000 MB. In binary (IEC) it is 1 GiB = 1,024 MiB. Hard drives are sold and priced in SI; your OS may display the identical drive using the binary count under an SI-looking label — hence the apparent "missing" space.
Why does my 500 GB drive show up as 465 GB in Windows?
Windows is dividing in binary (by 1,024 at each step, which is really GiB) but printing the label "GB". 500,000,000,000 bytes (the manufacturer's SI 500 GB) divided the binary way comes out to about 465. No storage is missing — Windows and the manufacturer are counting the same bytes with two different rulers.
What does macOS use?
Since macOS 10.6 Snow Leopard (2009), Finder reports sizes in true SI units: 1 GB = 1,000,000,000 bytes, matching the manufacturer's figure. The identical file that macOS calls 1 GB is one Windows would label roughly 0.93 GB — the Windows figure is about 6.9% smaller, because it is really counting gibibytes under a “GB” label.
How many bytes is 1 TB, and how big is the gap by then?
SI: 1 TB = 1,000,000,000,000 bytes exactly. Binary: 1 TiB = 1,099,511,627,776 bytes (240). The difference is 9.95% — by far the widest gap of any prefix step, since it's the decimal-vs-binary error compounded four times over.