There are two methods for determining the exact value of a measurement unit. You can calculate this using the International System of Units (SI) or the binary approach, which is used in most electronic devices.
They're actually two different conversions with a binary and SI difference of about 4.86%.
Converting Using SI
According to SI, the “Mega” prefix is either a multiplier of 10 or a million bits. In other words, SI is the decimal system or base 10 school.
In this case, 1 kilobyte equals 1,000 bytes, which makes 1 gigabyte equal 1,000 megabytes.
Converting Using Binary System
This method is mostly used with binary devices like computers, phones, tablets, and so on. The binary system is the base 2 school. Consequently, the binary mega prefix is 1,048,576 bits, making one gigabyte equal to 1,024 megabytes.
Consider the following representations to gain a better understanding of how the binary system works:
2^1 = 1
2^2 = 4
2^3 = 8
2^4 = 16
In binary, each new digit increases the number's value by one power of two. This explains that 2^10 equals 1,024.
In fact, this is one of the reasons your hard drive reads at a lower capacity than what you purchased. Assume you purchased a 250GB external hard drive and your computer only displays 232GB of free space for use.
Now, why is that? Technically, the drive has a capacity of 250,000,000,000 bytes. To determine the exact amount of storage space available, work your way backward through measuring units.
This means you'll begin by converting bytes to kilobytes, then kilobytes to megabytes, and megabytes to gigabytes. So, multiply the total storage capacity by 1,024 three times, and you'll get 232GB.
The larger the disc capacity, the greater the difference between what’s advertised and what the computer actually reads. A 1TB drive that’s supposed to read 1,000GB will only show 931GB.