Nowadays, we’ve got two official ways of measuring memory and storage. They can be measured via standard Metric Prefixes, such as Megabytes (MB) or Kilobytes (KB), or they can use the binary prefixes, such as Mibibytes (MiB) and Kibibytes(KiB). And yet, a lot of software seems to use the former, when referring to the latter. Why is that, exactly?

Well, part of the reason is that the official Binary SI prefixes didn’t exist until 2008, and were used to address growing ambiguities between them. Those ambiguities had been growing for decades.

From the outset, when Memory and storage were first developed in respect to computers, it became clear it would be necessary for some sort of notation to be used to measure the memory size and storage space, other than by directly indicating bytes.

Initially, Storage was measured in bits. A bit, of course, was a single element of data- a 0 or a 1. In order to represent other data and numbers, multiple bits would be utilized. While the sizes being in common discussion were small, bits were commonly referenced. In fact even early on there arose something of an ambiguity; often when discussing transfer rates and/or memory chip sizes, one would here "kilobit" or "megabit"; these would be 1,000 bits and 1,000 kilobits respectively, and were not base 2; however, when referring to either storage space or memory in terms of bytes, a kilobyte or a megabyte would be 1,024 bytes or 1,024 kilobits respective.

One of the simplest ways of organizing memory was using powers of two; this allowed a minimum of logic to access specific areas of the memory unit. Because the smallest addressable unit of storage was the byte, which were 8bits, it meant that most memory was manufactured to be a multiple of 1,024 bits, possible because it was the nearest power of 2 to 1,000 that was also divisible by 8. For the most part, rather than adhering strictly to the SI definitions for the prefixes, there was a industry convention that effective indicated that, within the context of computer storage, the SI prefixes were binary prefixes.

For Storage, for a time the same conveniences applied that resulted in total capacities measured in the same units. For example, A Single-sided 180K Floppy Diskette had 512 bytes per sector, 40 sectors a track, and 9 tracks a side.

A single sided 180K diskette had 512 bytes a sector, 40 sectors per track, and 9 tracks per side. That was 184320 Bytes. In today’s terms with the standardized binary prefixes, this would be 180KiB.

360K Diskettes had a similar arrangement but were double-sided. they were 368640 Bytes- again, Binary prefix was being used in advertising.

Same with 720 K 3-1/4" diskettes. 512 bytes/sector, 9 sectors per track, 80 tracks/side, two sides. That’s 737280 bytes. or 720KiB.

The IBM XT 5160 came with a drive advertised as 10MB in size. The disk has 512 bytes per sector, 306 cylinders, 4 heads, and 17 tracks. One cylinder is for diagnostic purposes and unusable. That gives us a CHS of 305/4/17. At 512 bytes/sector, that was 10,618,880 bytes of addressable space. (This was actually more than 10MiB as some defects were expected from the factory). The 20MB drive had a similar story as well. 615(-1 diag) cylinders, 4 heads, 17 sectors per track at 512 bytes a sector- 20.38MiB. The later 62MB drive was 940(-1 diag) cylinders, 8 heads, 17 sectors/track at 512 bytes/sector which gives ~62.36 MiB…

The "1.2MB" and "1.44MB" Floppy diskettes are when things started to get spitballed by marketing departments for ease of advertising and blazed an early trail for things to get even more misleading. The High density "1.2MB" diskettes were 512 bytes a sector, 15 sectors per track, 80 sectors per side, and double sided. That’s a total of 1,228,800 Bytes. or 1200 KiB, But they were then advertised as 1.2MB, Which is simply wrong altogether. It’s either ~1.7MiB, or it is ~1.23MB. it is NOT 1.2MB because that figure is determined by dividing the KiB by 1000 which doesn’t make sense. Same applies to "1.44MB" floppy diskettes, which are actually 1440KB due to having 18 sectors/track. (512 * 18 * 80 * 2=1474560 Bytes. That is either 1.47456MB, or 1.40625MiB, but was advertised as 1.44MB because it was 1440KiB (and presumably easier to write).

Hard drive manufacturers started to take it from there. First by rounding up a tiny bit- A 1987 Quantum LPS Prodrive advertised as 50MB was for example 49.87MB (752 cylinders, 8 heads, 17 sectors per track). I mean, OK- sure, 49.87 is a weird number to advertise I suppose…

it’s unclear when the first intentional and gross misrepresentation of HDD size was actually done where the SI Prefix definition was used to call a drive X MB. But, it was a gradual change. People started to accept the rounding and HDD manufacturers got more bold- eventually one of them released an X MB Drive that they KNEW full well people would interpret as X MiB, and when called out on it claimed they were using the "official SI Prefix" as if there wasn’t already a decades old de-facto standard in the industry regarding how storage was represented.

For the most part this confusion persisting forward is how we ended up with the official Binary Prefixes.

And yet- somewhat ironically – most OS software doesn’t use it. Microsoft Windows still uses the standard Prefixes. As I recall OSX provides for it as an option. Older Operating Systems and software will never use it as they won’t be updated.

The way I see it, HDD manufacturers have won. They are now selling Drives listed as "1TB" which are 930GiB, but because it’s 1,000,000,000,000 bytes or somewhere close, it’s totally cool because they are using the SI prefix.