Smartphones come in an array of memory sizes, with internal hard drives of currently up to 64GB.
We are often asked why a brand new device doesn't have the full capacity available for storage.
The first thing to note is that any device with internal SD memory like a smartphone (and all manner of items such as computers, external hard drives, etc) will always have slightly less space available for the user to access as storage of files than the capacity advertised.
This is because some of the space is used for the Operating System (OS) and the pre-installed and native applications. Continuing the example, if you were to analyse a brand new computer with a 320GB hard drive, you would see that some of the space has been already taken up and no matter how much you delete, you can never free that space up unless you were to format all partitions (different sections arranged so to keep areas of different purposes distinct), and then the computer wouldn't boot as there is no programming left to direct the hardware.
In relation to mobile phones, the more sophisticated smartphones generally have their memory classified as RAM or ROM and noted accordingly on their specifications.
RAM stands for Remote Access Memory and is where current processes that are running are stored and keep the data they need immediately available to them. This is the memory you see when you run any of the Task Managers showing you currently running apps and the available memory. It’s a completely different part of the phone from the Internal Memory (discussed below). Data stored in RAM requires constant power and does not survive a power cycle of the phone (volatile) and becomes constrained when there are more and/or larger processes running simultaneously that the device has the capability to deal wtih.
ROM stands for Read Only Memory and it’s essentially computer memory that does not require power to store it’s data (non-volatile). In the sense of a smartphone, it’s the Internal Memory where the OS plus the native apps and user storage is found. This is the memory capacity that we are addressing in this post.
Basically the ROM is Flash Memory partitioned into two sections, one for the OS and the rest for apps to use. So, the OS partition essentially is true ROM, unless you root the phone. The software that groups like xda-developers make available are called ROMs because they’re a ROM Image. The space within the Internal Memory that isn't used by the ROM is where the user content is stored and is the total capacity minus the OS ROM size.
This means that your advertised 16GB will be broken up into disctinct partitions, generally one for the essentials and the content that comes on the phone and another that you can store your own content on. In most cases the ROM allocated towards the OS and native apps is about 10-20% of the total capacity, so you might have have say only 14 of the 16GB available for your apps, music, photo's and files. However there are variations in the ways that manufacturers configure this element of a device and in come instances they may partition the Internal Memory 50/50 between the OS and the user, so you might only have 8GB for file storage and the other 8GB for the OS and your apps (both pre-installed and user-added), etc. Beware that these memory configurations vary per device so there may be a significant difference in the amount of space available for your files in 2 devices of the same memory capacity.
If you find that your current Internal Memory isn't enough, you can extend this with a mobile memory card, but if you are running out of ROM or RAM you'll need lose some apps, or consider a new mobile phone.