Know that quantities of bytes can be described using prefixes.

The idea

Byte quantities are often written with prefixes so large numbers stay readable. In this specification the prefixes are decimal (powers of 1000): kilo means 1000 bytes, mega means 1 000 000 bytes, giga means 1 000 000 000 bytes and tera means 1 000 000 000 000 bytes. To convert a number of bytes into a larger unit, divide by the size of that unit; to convert a larger unit into bytes, multiply. For example, 5000 bytes ÷ 1000 = 5 kB, and 2 MB × 1 000 000 = 2 000 000 bytes. Always state the unit with the number, and check the direction of the conversion by asking whether the answer should be a bigger or smaller number.

What earns the marks

• Recognises that prefixes such as kilo, mega, giga and tera describe quantities of bytes.
• States the decimal values: 1 kB = 1000 bytes, 1 MB = 1 000 000 bytes, 1 GB = 1 000 000 000 bytes, 1 TB = 1 000 000 000 000 bytes.
• Converts bytes to a larger prefixed unit by dividing by the appropriate power of 1000.
• Converts a prefixed unit to bytes by multiplying by the appropriate power of 1000.
• Checks a conversion by reasoning whether the numerical value should increase or decrease.
• Gives the unit alongside the numerical answer.

Common mistakes and how to put them right

• Multiplying when converting bytes to a larger prefixed unit.

  Put it right: Dividing by 1000, 1 000 000 and so on gives the smaller number expected for a larger unit.

• Treating kilo as 1024 in this specification.

  Put it right: The specification uses decimal prefixes, so 1 kB = 1000 bytes.

• Dropping the unit from the answer.

  Put it right: Write the number with its unit, for example 5 kB rather than 5.

Exam tips

• Write the conversion factor before calculating, for example 1 MB = 1 000 000 bytes.
• Predict whether the answer should be larger or smaller than the starting value, then check your arithmetic against that prediction.
• Show the division or multiplication explicitly so a slip in powers of ten is easy to spot.