Using standardized SI prefixes

James "Doc" Livingston doclivingston at
Wed Jun 13 23:03:02 UTC 2007

On Wed, 2007-06-13 at 15:01 +0100, Alex Jones wrote:
> On Wed, 2007-06-13 at 14:29 +0100, Scott James Remnant wrote:
> > Without the binary unit to consider, when we quote a drive as 1TB, we
> > know that it has *at least* 1,000,000,000,000 bytes available.
> > Depending on the drive, it may have anywhere between this and
> > 1,099,511,627,776 bytes available.  It's actually more likely to have
> > something strange like 1,024,000,000,000 available.
> 10% error is no good for me. You can continue to play the "at least"
> card, but what about when it's more important if it is "at most"
> something? And seeing as this error only goes up exponentially, at which
> prefix do you draw the line and say "no more"?

You'll get error anyway - a "500Gb disk" doesn't necessarily contain
500x10^9 bytes, 500x2^30 bytes or any combination of 10^m and 2^n. Drive
manufacturers build drives make them with densities and sizes that are
convenient to them, and round down to the nearest "nice number" for
marketing purposes. A "500Gb" drive could have 512,345,678,900 bytes on
it for all we know.

> And no-one uses floppy disks any more. Let's just bury them all and
> forget about them. :D
> > I see no problem with this "1TB" quote being approximate.  It's rounded
> > anyway.  If you really want to know how many bytes are available, you
> > can use this great unit called the "byte" which is accurate and not
> > subject to change[0].
> 1 TB is not rounded. It means precisely 1 × 10^12 bytes, no more and no
> less. If they want to actually put 1.024 TB on the disk then they can
> say 1 TB (approx.) like any other industry (detergent, bacon, etc.).

How many other industries do this? If I buy a 500g pack of bacon, I
don't get 500g - I get around 500g, close enough that the appropriate
consumer trading authority doesn't come and have words with them. Very
few things I ever buy have "approx" mentioned with how much I get.


"Too many errors on one line (make fewer)" -- MPW C error message

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