How big is that

How big is a Megabyte?

Many people ask questions like "How many bytes are there in a megabyte?".
There are actually two answers. I hope this note can help you with these answers.

Firstly; the pure mathematical answer:
Kilo, Mega, etc. are all powers of 10. For example Kilo is 10 to the power of 3 = 1,000 (one thousand), Mega is 10 to the power of 6 = 1,000,000 (one million), The power of ten tells you how many 0's to put after the 1.
The table below show all the terms used up to 10 to the power of (**) 21.


Term	Letter	10**	Written fully:
Kilo	K	3	1,000
Mega	M	6	1,000,000
Giga	G	9	1,000,000,000
Tera	T	12	1,000,000,000,000
Peta	P	15	1,000,000,000,000,000
Zeta	Z	18	1,000,000,000,000,000,000
Yotta	Y	21	1,000,000,000,000,000,000,000

For example 1 Kgm of sugar is the same as 1000 gm of sugar.

Secondly, there is the computer answer:
It just so happens that with ten binary digits (bits) you can count to 1024, which is just a fraction higher than a thousand.
Because of this all people working with computers started to use the term "Kilo" to refer to 2**10 = 1024, i.e. the maximum for ten bits. This goes on with "Mega" referring to 2**20 (20 bits) = 1024 * 1024 = 1,048,576.
The table below show all the terms used up to 2 to the power of (**) 70.


Term	Letter	2**	Written fully:
Kilo	K	10	1,024
Mega	M	20	1,048,576
Giga	G	30	1,073,741,824
Tera	T	40	1,099,511,627,776
Peta	P	50	1,125,899,906,842,624
Zeta	Z	60	1,152,921,504,606,846,976
Yotta	Y	70	1,180,591,620,717,411,303,424

As an example, 1Mbyte is actually not 1,000,000 bytes but 1,048,576 bytes; quite a bit more.
It should be noted that not all people working in the computer industry are consistent with these figures. For instance I have seen people talking about a Mbyte as 1,024,000 bytes.
So if you have just acquired a 20Gbyte disk, it may have a capacity anywhere between 20,000,000,000 bytes up to 21,474,836,480 bytes. A difference of just over 7%.

Updated 09-Sep-2022.