Binary and decimal data sizes explained
Why 1 GB can mean 1024 MB or 1000 MB, how bytes differ from bits, and why a 1 TB drive shows about 931 GB in your system.
Two ways to count a gigabyte
Digital storage grows in steps, and there are two conventions for how big each step is. The binary convention multiplies by 1024 at every level, so a kilobyte is 1024 bytes, a megabyte is 1024 kilobytes, and a gigabyte is 1024 megabytes. The decimal convention multiplies by 1000 instead, matching the metric prefixes used elsewhere in science. This converter follows the binary convention because that is what desktop and phone operating systems display.
Bytes are not bits
Alongside the 1024 versus 1000 question sits an easier one to mix up: the difference between a byte and a bit. A byte is eight bits, and by long habit a capital B means bytes while a lowercase b means bits. File sizes are quoted in bytes, so megabytes and gigabytes here are byte units. Internet speeds are usually quoted in bits, which is why a connection sold as 100 megabits per second downloads at roughly 12.5 megabytes per second.
Why a labeled drive looks smaller
A drive sold as 1 TB is labeled with decimal terabytes, meaning one trillion bytes. Your operating system divides that by 1024 three times to reach binary gigabytes, giving about 931 GB, and a 1 TB figure becomes roughly 0.91 TB in binary. Nothing is lost or hidden; the drive simply holds the bytes it promised, counted under a different convention. The same effect is why a 500 GB drive shows near 465 GB.
The IEC names that remove the doubt
To end the confusion, the IEC introduced explicit binary names: kibibyte, mebibyte and gibibyte, written KiB, MiB and GiB, each a power of 1024. Under that scheme MB and GB are reserved for the decimal powers of 1000. Many systems still say MB and GB while meaning the binary amounts, which is the historical reason a single label can point at two slightly different sizes.