The factor: 1,024, not 1,000
1 gigabyte equals exactly 1,024 megabytes in this convention — not 1,000. Likewise, 1 terabyte equals 1,024 gigabytes, and 1 megabyte equals 1,024 kilobytes. Each step up a size prefix multiplies by 1,024, not a round 1,000.
Why 1,024 specifically
1,024 is 2¹⁰ — a power of 2, which fits naturally with how computers represent numbers in binary. This "binary" convention (sometimes labeled KiB, MiB, GiB for clarity) is what operating systems and file managers have traditionally used to report file and storage sizes.
What "1 byte = 8 bits" adds on top
Bits and bytes are a separate, smaller-scale relationship layered underneath the kilobyte-and-up prefixes: 1 byte is always exactly 8 bits, regardless of which size-prefix convention (1024 or 1000-based) is being used above it.
Why this convention doesn't apply everywhere
Not every data-related unit uses this 1,024-based convention — network transfer rates (like Mbps) conventionally use a 1,000-based decimal convention instead, a genuine difference explored in the companion article on why storage and transfer rate use different bases.