From the keyboard of Dr Jan

As you may know, the prefix mega- is abbreviated with an upper case ‘M’. So, for example, 5 megabytes can be written as 5MB. A single megabyte is either 1,000,000 or 1,048,576 bytes.

From the Wikipedia:

In computing, mega- can sometimes denote 1,048,576 (2²⁰) of information units (example: a megabyte, a megaword), but can denote 1,000,000 (10⁶) of other quantities, for example, transfer rates: 1 megabit/s = 1,000,000 bit/s.

The prefix mebi- has been suggested as a prefix for 2²⁰ to avoid ambiguity, but is yet to be widely used.

So, rather cheekily, disk drive manufacturers always use megabyte to mean 1,000,000 bytes, which makes their drives appear larger. A 500MB drive will have 500,000,000 bytes, or about 4.8 mebibytes.

Conversely, the milli- prefix denotes thousandths (10^-3) and has the abbreviation lower case ‘m’. 5mA refers to 5 milliamps, or 5 thousandths of an amp. Amps get an upper-case ‘A’ because they’re named after Andre-Marie Ampere.

Under no circumstances should you confuse the two! It annoys me when I see computers advertised with 512mb of memory. 512 millibytes is just over half a byte. OK, it’s fairly obvious in this case that they meant to use ’512MB’, but all it says to me is they either didn’t know or, worse, didn’t care about the difference between mega- and milli-.

Incidentally, a byte originally referred to a collection of bits, but in these modern times everyone seems to have agreed that a byte is made up of exactly 8 bits. Somewhat amusingly, half a byte or 4 bits can be referred to as a nybble. Beyond that, 2 bits are a morsel, although I’ve never really heard that used in real life.

So, 512mb is just over a nybble

UPDATE: Oh my goodness, it gets worse! It turns out that upper case ‘B’ refers to bytes, lower case ‘b’ is for bits. Which means that 512mb is 512 millibits, or just over half a bit!

Damn! How can I rant about people confusing M with m when I’m confusing B with b myself?

As always, it’s a learning experience