KB or KiB, the two meanings of a kilobyte
Data sizes have two competing rulebooks. Decimal units use metric prefixes, so each step is 1,000 times the last, and this is how storage and broadband are marketed. Binary units step by 1,024 (2 to the power of 10), because memory is addressed in powers of two, and these are properly written KiB, MiB, GiB and TiB (kibibytes, mebibytes and so on):
The tool uses decimal by default; tick the binary box to switch everything, labels included, to 1,024-based units. Whichever mode you pick, every conversion goes through bytes, the one unit both systems agree on.
Worked examples
Why your drive looks smaller than advertised
Nothing is missing and nobody short-changed you. The manufacturer counts your 1 TB drive in decimal: exactly 1,000,000,000,000 bytes, which is the legally standard meaning of the prefix. Windows then takes that same number of bytes, divides by 1,073,741,824, and labels the answer "GB" even though it has really calculated GiB. The result, 931, looks like a 7% shortfall but is just the same quantity measured with two different rulers. macOS switched to decimal reporting years ago, which is why the same drive shows "1 TB" on a Mac and "931 GB" on a PC. The gap grows with each prefix: about 2.4% at KB, 4.9% at MB, 7.4% at GB and just under 10% at TB.
A second unit clash catches people out when downloading: broadband speeds are sold in megabits per second (Mbps), while file sizes are measured in bytes, and there are 8 bits in a byte. A 100 Mbps connection therefore moves at most 12.5 MB of file per second, so a 1 GB download takes at least 80 seconds even on a connection that sounds like it should manage it in 10. Watch the capitalisation: the little b in Mbps means bits, the big B in MB/s means bytes.
Where does binary genuinely rule? Memory. RAM is built and addressed in powers of two, so an "8 GB" memory module really is 8 GiB (8,589,934,592 bytes), with no marketing gap at all. Storage, file sizes and network speeds went decimal; memory never did. If a number refuses to add up, converting everything to plain bytes, which this tool always shows, settles the argument.
Sizes worth knowing
- A plain text email: a few KB
- A smartphone photo: 2 to 5 MB
- An MP3 track: around 8 MB
- A DVD: 4.7 GB (4.38 GiB)
- An hour of HD streaming: roughly 3 GB
- A modern console game: often 50 to 150 GB
If you are trying to make files smaller rather than just measure them, our image compressor shrinks photos in your browser, and the Base64 tool shows how encoding inflates file sizes by about a third.
Frequently asked questions
How many MB are in a GB?
In decimal units, 1,000 MB make a GB, and that is how storage is sold and how this tool works by default. In binary units, 1,024 MiB make a GiB. Tick the binary box if you want the 1024-based figures.
What is the difference between KB and KiB?
KB (kilobyte) officially means 1,000 bytes, using the metric prefix. KiB (kibibyte) means 1,024 bytes, the power-of-two unit computers naturally use. Historically KB was used loosely for both, which is the root of most data size confusion.
Why does my 1 TB drive show as 931 GB?
The manufacturer counts in decimal: 1 TB = 1,000,000,000,000 bytes. Windows divides that same number of bytes by 1,073,741,824 (binary) and labels the result GB, giving 931. No space is missing, the two sides are just using different definitions of the same unit names.
Should I use decimal or binary units?
Decimal matches how storage and broadband are marketed, so it is the default here. Binary matches what Windows reports for drives and what memory (RAM) uses. If you are comparing a file size on screen with an advertised capacity, convert both through bytes, which is exactly what this tool does.
How big is a typical file?
As a rough guide: a plain email is a few KB, a smartphone photo is 2 to 5 MB, an hour of streamed HD video is around 3 GB, and modern games can exceed 100 GB. A 4.7 GB DVD holds about 4.38 GiB in binary terms.