Gigabyte or Gibibyte (1000 or 1024)?

Question

This may be a duplicate and I apologies if that is so but I really want a definitive answer as that seems to change depending upon where I look.

Is it acceptable to say that a gigabyte is 1024 megabytes or should it be said that it is 1000 megabytes? I am taking computer science at GCSE and a typical exam question could be how many bytes in a kilobyte and I believe the exam board, AQA, has the answer for such a question as 1024 not 1000. How is this? Are both correct? Which one should I go with?

Thanks in advance- this has got me rather bamboozled!

Answer 1

The sad fact is that it depends on who you ask. But computer terminology is slowly being aligned with normal terminology, in which kilo is 10³ (1,000), mega is 10⁶ (1,000,000), and giga is 10⁹ (1,000,000,000).

This is reflected in the International System of Quantities and the International Electrotechnical Commission, which define gigabyte as 10⁹ and use gibibyte for the computer-specific 1024 x 1024 x 1024 value.

The reason it "depends who you ask," is that for many years, specifically in relation to "bytes" of storage, the prefixes kilo, mega, and giga meant 1024, 1024², and 1024³. But that flies in the face of normal convention with regard to these prefixes. So again, computer terminology is being aligned with non-computer terminology.

Answer 2

The term gigabyte is commonly used to mean either 1000³ bytes or 1024³ bytes depending on the context. Disk manufacturers prefer the decimal term while memory manufacturers use the binary.

Decimal definition

• 1 GB = 1,000,000,000 bytes (= 1000³ B = 10⁹ B)

Based on powers of 10, this definition uses the prefix as defined in the International System of Units (SI). This is the recommended definition by the International Electrotechnical Commission (IEC). This definition is used in networking contexts and most storage media, particularly hard drives, flash-based storage, and DVDs, and is also consistent with the other uses of the SI prefix in computing, such as CPU clock speeds or measures of performance.

Binary definition

• 1 GiB = 1,073,741,824 bytes (= 1024³ B = 2³⁰ B).

The binary definition uses powers of the base 2, as is the architectural principle of binary computers. This usage is widely promulgated by some operating systems, such as Microsoft Windows in reference to computer memory (e.g., RAM). This definition is synonymous with the unambiguous unit gibibyte.

The difference between units based on decimal and binary prefixes increases as a semi-logarithmic (linear-log) function—for example, the decimal kilobyte value is nearly 98% of the kibibyte, a megabyte is under 96% of a mebibyte, and a gigabyte is just over 93% of a gibibyte value. This means that a 300 GB (279 GiB) hard disk might be indicated variously as 300 GB, 279 GB or 279 GiB, depending on the operating system.

The Wikipedia article https://en.wikipedia.org/wiki/Gigabyte has a good writeup of the confusion surrounding the usage of the term