Your smartphone doesn't give you the full amount of memory marked on the case. The reason being? The Operating Systems take up way too much space! Different phones will leave you with different amounts of storage.
Check this infographic from
Which?, to find out which are the most generous phones:
Among 16GB smartphones, the iPhone 5C ends up leaving the most space open for the user at 79%. Bottom of the barrel? Samsung's Galaxy S4, because it is loaded with so much bloatware that you only get 54% of the 16GB offered.