How does FreeBSD calculate disk sizes

Alexander Marx mad-ml at
Mon Nov 3 01:52:06 PST 2003

Odhiambo Washington wrote:
> Hello users,
> I have a disk which is actually 72GB. 2GB has been used as swap while
> the rest was given to /.
> Can someone explain to me what I could be missing here, because what
> I am seeing isn't what I expect. Perhaps it's just right while I am
> the dumb one. Why isn't the whole size reported?

first of all, a "72GB harddisk" isn't actually a 72GB harddisk; this might
sound funny, but harddisk manufacturers are engineering people and they
are usually thinking in terms of 1000s

so your disk actually only holds 72000000000 bytes (~69GB) ...
which is perfectly consistent with your fdisk output.

further, reading tunefs(8) you will find, that per default your filesystem
reserves some space (8%) for root ...

: -m minfree
:             Specify the percentage of space held back from normal users; the
:             minimum free space threshold.  The default value used is 8%.

.. subtracting 8% from the remaining 69GB roughly gives 64GB.

so your disk is just fine.


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