[Nexus 5 Q&A, Help & Troubleshooting] danarama:
32 GB Nexus isn't 32 GB. 16 GB Nexus isn't 16 GB

Here is how it works.

A true GB is 1024 MB, which is 1024 KB, which is 1024 B. This is how Software (including operating systems) report a true GB (Now referred to as GiB).

Hardware manufacturers however, refer to a GB as 1000 MB, which is 1000 KB, which is 1000 B. So there is a disparity here that gets bigger, the higher the number.

So you need to employ mathematics to work out the TRUE capacity of the Nexus 5.

Take your 32 GB reported hardware, and multiply it by 1000 a total of 3 times. Then, take the result and divide it by 1024 a total of 3 times. This is the total true capacity

32 x 1000 MB x 1000 KB x 1000 B = 32,000,000,000 B

32,000,000,000 B / 1024 B / 1024 MKB / 1024 MB = 29.8 GB

So in summary, any SD card, SSD, Device, HDD that reports to have "32 GB" capacity, it is only ever 29.8 GB (or 29.8 GiB) capacity.

So now it starts to add up a bit more. Then obviously there is a recovery partition, cache partition, System partition and more..... which is excluded from the total "storage"

Here are the main partitions (excluding /data which was 12.6GB) for the N5 (taken from a 16GB) - use "df" from terminal

/system 1009.3M

/cache 689.8M

/persist 15.8M

/firmware 64.0M

However, there are technically 29 partitions, they obviously don't add up to much between the others - use "ls -lR /dev" from terminal

lrwxrwxrwx root root 1970-01-15 02:15 DDR -> /dev/block/mmcblk0p24
lrwxrwxrwx root root 1970-01-15 02:15 aboot -> /dev/block/mmcblk0p6
lrwxrwxrwx root root 1970-01-15 02:15 abootb -> /dev/block/mmcblk0p11
lrwxrwxrwx root root 1970-01-15 02:15 boot -> /dev/block/mmcblk0p19
lrwxrwxrwx root root 1970-01-15 02:15 cache -> /dev/block/mmcblk0p27
lrwxrwxrwx root root 1970-01-15 02:15 crypto -> /dev/block/mmcblk0p26
lrwxrwxrwx root root 1970-01-15 02:15 fsc -> /dev/block/mmcblk0p22
lrwxrwxrwx root root 1970-01-15 02:15 fsg -> /dev/block/mmcblk0p21
lrwxrwxrwx root root 1970-01-15 02:15 grow -> /dev/block/mmcblk0p29
lrwxrwxrwx root root 1970-01-15 02:15 imgdata -> /dev/block/mmcblk0p17
lrwxrwxrwx root root 1970-01-15 02:15 laf -> /dev/block/mmcblk0p18
lrwxrwxrwx root root 1970-01-15 02:15 metadata -> /dev/block/mmcblk0p14
lrwxrwxrwx root root 1970-01-15 02:15 misc -> /dev/block/mmcblk0p15
lrwxrwxrwx root root 1970-01-15 02:15 modem -> /dev/block/mmcblk0p1
lrwxrwxrwx root root 1970-01-15 02:15 modemst1 -> /dev/block/mmcblk0p12
lrwxrwxrwx root root 1970-01-15 02:15 modemst2 -> /dev/block/mmcblk0p13
lrwxrwxrwx root root 1970-01-15 02:15 pad -> /dev/block/mmcblk0p7
lrwxrwxrwx root root 1970-01-15 02:15 persist -> /dev/block/mmcblk0p16
lrwxrwxrwx root root 1970-01-15 02:15 recovery -> /dev/block/mmcblk0p20
lrwxrwxrwx root root 1970-01-15 02:15 rpm -> /dev/block/mmcblk0p3
lrwxrwxrwx root root 1970-01-15 02:15 rpmb -> /dev/block/mmcblk0p10
lrwxrwxrwx root root 1970-01-15 02:15 sbl1 -> /dev/block/mmcblk0p2
lrwxrwxrwx root root 1970-01-15 02:15 sbl1b -> /dev/block/mmcblk0p8
lrwxrwxrwx root root 1970-01-15 02:15 sdi -> /dev/block/mmcblk0p5
lrwxrwxrwx root root 1970-01-15 02:15 ssd -> /dev/block/mmcblk0p23
lrwxrwxrwx root root 1970-01-15 02:15 system -> /dev/block/mmcblk0p25
lrwxrwxrwx root root 1970-01-15 02:15 tz -> /dev/block/mmcblk0p4
lrwxrwxrwx root root 1970-01-15 02:15 tzb -> /dev/block/mmcblk0p9
lrwxrwxrwx root root 1970-01-15 02:15 userdata -> /dev/block/mmcblk0p28

USABLE space for the user is:

26.7 GB for the 32

12.55 GB for the 16.