What unit is "data transmission speed" measured in?

Data transmission speed is usually measured in bits per second and its derived units -- kilobits, megabits and gigabits per second. Sometimes it is expressed in bytes; a byte equals eight bits.

Data transmission speeds are often referred to as "bit rates" when talking about hardware, such as a network interface card.

Data units are traditionally binary units; thus, they are multiplied by powers of two, making 1 kilobit equal to 2^10 (1,024) bits, 1 megabit equal to 2^20 (1,048,576) bits, and so on. However, because units in the International System of Units are decimal, that is, multiplied by powers of 10, kilobits are defined as 10^3 (1,000) bits, megabits as 10^6 (1,000,000) bits, and so on.