In the decimal (SI) system, 1 kilobyte (KB) = 1,000 bytes. In the binary system used by operating systems, 1 kibibyte (KiB) = 1,024 bytes. This difference is why a 1 GB hard drive may show slightly fewer than 1,000 MB when connected to a computer that uses binary prefixes.

A megabyte (MB) equals 8 megabits (Mb). Internet speed is typically measured in megabits per second (Mbps), while file sizes are measured in megabytes. So a 100 Mbps connection can theoretically download a 100 MB file in about 8 seconds.

One terabyte (TB) equals 1,000 gigabytes (GB) in decimal notation, or 1,024 gibibytes (GiB) in binary. A modern 1 TB hard drive can hold roughly 250,000 photos at 4 MB each, or about 500 hours of standard-definition video.

The units progress as: byte → kilobyte (KB) → megabyte (MB) → gigabyte (GB) → terabyte (TB) → petabyte (PB) → exabyte (EB) → zettabyte (ZB) → yottabyte (YB). Each step is 1,000× larger in the decimal system. Global internet traffic is currently measured in petabytes per second.

Hard drive manufacturers use the decimal definition (1 GB = 1,000,000,000 bytes), but operating systems like Windows display capacity using the binary definition (1 GB = 1,073,741,824 bytes). A 500 GB drive actually contains 500,000,000,000 bytes, which Windows reports as approximately 465 GiB.