I installed 16 GB ram but only 5.88 is usable

[Anunsha]

RAM was not used completely. more than half of ram was not even shown in the task manager but its mentioned as 16 GB in specifications

[BeemerBiker]

You may have 16gb of memory but 8gb might be all that is available.

There are a lot of reasons this can happen as explained here

It is also possible the wrong memory is being used. You may have memory of different ranks (mis-matched memory). The memory may be in the wrong slots, especially on desktop system.

An easy way to find what memory you have is to run the wmic app from the windows command prompt. Bring up the window CMD prompt and enter the first line to see what memory you have. The second line may give the maximum memory possible based on the chipset. The number of memory sockets limit what can be added.

wmic memorychip get devicelocator, capacity, speed, MemoryType, formfactor, partnumber, manufacturer

wmic memphysical get maxcapacityEx, MemoryDevices

Should look something like the following. 

Please provide the Product ID or exact model name for better assistance. Preferably the full product ID including the characters after the # symbol.