Welcoming back a repaired computer is almost as good as bringing your partner home from hospital after successful treatment.

When my G5 DP suffered a graphics card failure, it returned with a sparkling new ATI Radeon 9800 Pro Mac Special Edition graphics card, and I took the opportunity to add a second 250 GB serial ATA hard disk, onto which I could mirror the first using Mike Bombich’s superb Carbon Copy Cloner (which is still going strong, unlike my PowerMac G5). Only neither hard disk actually provided 250 gigabytes of space, of course.

Just as the 2 TB drive inside my current iMac provides 1.82 terabytes.

A little of their space understandably gets consumed by overheads, like the partition map (which sets out which parts of the disk are used for what), but most of the apparent deficit is accounted for by the non-standard ways in which we measure computer storage capacity. Although one kilometre is exactly 1000 metres, one kilobyte can be either 1000 bytes, or 1024, depending on who is doing the counting.

The hard disk vendor, in promising me the capacity of their product, is actually offering me 250 000 000 000 bytes of space before formatting. They call that 250 gigabytes in the same way that I could quaintly refer to 250 000 000 000 metres as 250 gigametres. If you’re a bit rusty on units and the like, you’ll enjoy hearing from the horse’s mouth from the Bureau International des Poids et Mesures, clearly a truly international body.

When older versions of Disk Utility or the Finder look at that hard disk, they used to consider that there were 1024 bytes in a kilobyte, and so on, and thus that my 250 gigabyte disk actually provided only 232.83 gigabytes of space before formatting – a 7% fall in capacity. The same principle applied to blank DVD-R media, which proudly proclaim they are of 4.7 gigabyte storage capacity, but can only actually take 4.377 ‘real’ gigabytes of data.

Apart from leaving us with the feeling of being slightly shortchanged, it could lead to embarassing shortfalls if you are pushing the limit of the capacity of your medium.

These days Disk Utility, the Finder, and everything else have forsaken the factor of 1024, and use 1000, keeping capacities in kilter.

This is reminiscent of another ploy to exaggerate measurements without actually lying, in the specification of monitors and TV screens. The first time that you tried to measure the sides of a 21 inch monitor, only to find that this referred to the screen diagonal, you probably felt as cheated as when you realised that your 250 GB disk is only 232.83 GB in size. Monitor dimensions most probably hark back to the early days of CRT televisions, when vendors were desperate to do anything that might make them sound larger, and ‘colour’ referred to the tone of the veneered case.

Officially, according to NIST, our Macs should have referred to ‘real’ storage units as (e.g.) ‘gibibytes’, the large ones based on multiples of 1024 (2^10) rather than 1000 (10^3), whilst the disk vendor was correct in referring to 1 000 000 000 bytes as one ‘gigabyte’. We should perhaps be grateful for the demise of the 3.5 inch ‘1.44 megabyte’ floppy disk, because it used a third definition for a megabyte, being 1 024 000 bytes, an inexcusable bastardisation of the other two systems.

Meanwhile back in the real world, I challenge you to find any written use of the units named ‘gibibytes’, ‘mebibytes’, or ‘kibibytes’, which seem to exist only in the imagination of some metrologists who care not for euphonic English.

Sadly it seems that we missed the chance to standardise on 1024 as the multiplier, and went mealy-mouthed for the option of 1000. I still feel slightly cheated when initialising a new hard disk.

Updated from the original, which was first published in MacUser volume 20 issue 19, 2004.