# Re: Using standardized SI prefixes

```On Wed, 13 Jun 2007, 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"?

And no-one uses floppy disks any more. Let's just bury them all and

```
```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.).
```
```
```
1 TB has only one significant digit. It would be silly to think that it was an exact measurement, at least in fields I am familiar with. ;) No one I know would think 1km is as precisely measured as 1.00000000000000000km.
```
```
But, just because it is approximate, doesn't mean it isn't also ambigouous. :) 1 TB could mean between 500000000000 and 1499999999999 bytes, between 549755813888 and 1649267441663 bytes, or even between 500000000000 and 1499999999999.99... bels. :)
```
```
So, if you want the exact number of bytes, don't round it off, and if you do round it off, don't be surprised if the rounding is ambiguous, because the units are not SI units, and the prefixes may or may not be. Just don't use prefixes when not rounding.
```
I wonder, do people feel as storngly about exactly how many tablespoons
1 TT is?

Ivan```