How it works
Convert file and drive sizes between decimal (SI) units, where each step is 1,000, and binary (IEC) units, where each step is 1,024. One byte is 8 bits.
Drive makers use decimal units, so a 1 TB drive holds 1,000,000,000,000 bytes. Windows reports sizes in binary units but labels them KB, MB and GB, which is why that drive shows up as about 931 "GB" (really 931 GiB). macOS has used decimal units since 2009.
Formula
Decimal: 1 kB = 1,000 B, 1 MB = 1,000 kB … Binary: 1 KiB = 1,024 B, 1 MiB = 1,024 KiB …
Questions people ask
Is a gigabyte 1,000 or 1,024 megabytes?
By the SI and IEC standards, a gigabyte (GB) is 1,000 megabytes and a gibibyte (GiB) is 1,024 mebibytes. Some software still uses "GB" to mean GiB.
Why does my new hard drive show less space than advertised?
Because it is labeled in decimal units but shown in binary units. 1 TB = 10¹² bytes, which is about 0.909 TiB or 931 GiB. Formatting also uses a little space.
How many bits are in a byte?
8 bits. Network speeds are usually given in bits per second and file sizes in bytes, so divide a bit rate by 8 to get bytes.