Units of information: b represents bit

The idea

In units of information the lower-case letter b is the symbol for a bit, a single binary digit that is 0 or 1. The capital letter B is used for a byte, a group of 8 bits, so the case of the letter changes the meaning and the size by a factor of eight. This distinction appears in data rates and storage sizes: a connection quoted in Mb/s transfers megabits each second, while a file size in MB uses megabytes. To avoid errors, read the symbol carefully and convert consistently — 1 B = 8 b, so 1 MB = 8 Mb. When a question gives a rate in Mb/s and a file size in MB, convert one to the other before dividing, keeping the units labelled at every step.

What earns the marks

• The lower-case symbol b represents a bit, a single binary digit.
• The capital symbol B represents a byte, a group of 8 bits.
• The case of the symbol changes the unit, so b and B are not interchangeable.
• 1 byte equals 8 bits, so a size in bytes converts to bits by multiplying by 8.
• Data rates such as Mb/s use bits per second, whereas storage sizes such as MB use bytes.

Common mistakes and how to put them right

• Treating b and B as the same unit.

  Put it right: b is a bit and B is a byte, differing by a factor of 8.

• Assuming a rate in Mb/s equals the same number in MB/s.

  Put it right: Divide the bit rate by 8 to get bytes per second.

• Mixing units in a calculation without converting.

  Put it right: Convert to a single unit, such as bits, before dividing.

Exam tips

• Check the case of the symbol before calculating; b means bit and B means byte.
• Convert between bits and bytes by multiplying or dividing by 8, and keep units written beside each value.
• For rate-and-size problems, convert the file size to bits to match a bit-rate, then divide.