Question

What the unit to calculate computer memory ?​

Answer 1

SI prefixes are defined as powers of ten, actually of 1000 after you get past kilo-. Per the SI Brochure they are not to be used for powers of 1024. Computer architecture usually has binary address lines and max memory (RAM) is some power of 2, often redefined in powers of 1024. The IEC introduced a series of binary prefixes with different but similar symbols to define powers of 1024.

Thus, 1 GB = 10^9 bytes

but 1 GiB = 1024^3 bytes (the written out form is gibi a contraction of giga and binary. The highest currectly defined is exbi (Ei), meaning 1024^6.

Note that SI prefixes can be used with bytes if and only if they actually mean their powers of ten. For communications speed, disk drive capacity, etc. they normally do. RAM, flash memory, etc generally have a binary structure of addressing and should use binary prefixes or use decimal prefixes correctly.

1 GiB = 1.073 741 824 GB, which would be a little messy to write out correctly.