Binary prefixes (Cambridge International AS Computer Science)

Exam board Cambridge International · ASUpdated 2026-05-05

Spec point — 1.1.1

A binary prefix is a unit prefix used to indicate multiples of bytes in binary
Consider the word kilobyte, "kilo" is the prefix
The byte is the standard unit used for measuring data storage, so there needs to be a way of expressing bytes in larger multiples

This system relies on the assumption that 1 kilo = 1000
This assumption is based on the denary (base 10) number system
- E.g. a 1 GB hard drive can store 1 x 10⁹bytes

However, computers use the binary (base 2) number system so the denary system is technically inaccurate when describing storage
To be precise, expressing multiples of bytes is done using binary prefixes:

Notice the prefixes change depending on the system being used, e.g. kilo (denary) vs kibi(binary)

The importance of the system being used depends on how precise you need to be
When precision is required (e.g. identifying RAM), binary prefixes should be used
- E.g. 16 GiB RAM can store 16 × 2³⁰ bytes (17,179,869,184 bytes)
When a rough estimate is acceptable (e.g. describing storage device capacity), denary prefixes are commonly used
- E.g. a 16 GB memory stick can store 16 × 10⁹ bytes (16,000,000,000 bytes)

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Denary unit

Equivalent size (bytes)

1 kilobyte (1 KB)

1000

1 megabyte (1 MB)

1,000,000

1 gigabyte (1 GB)

1,000,000,000

1 terabyte (1 TB)

1,000,000,000,000

1 petabyte (1 PB)

1,000,000,000,000,000

Binary unit

Number of bytes (base 2)

Equivalent size (base 10)

1 kibibyte (1 KiB)

2¹⁰

1024

1 mebibyte (1 MiB)

2²⁰

1,048,576

1 gibibyte (1 GiB)

2³⁰

1,073,741,824

1 tebibyte (1 TiB)

2⁴⁰

1,099,511,627,776

1 pebibyte (1 PiB)

2⁵⁰

1,125,899,906,842,624