Only 12GB of ram showing when I have 16GB

Make sure the RAM is properly plugged in first. If it doesn't work, Take out all the RAM and clean the slots with a blower or compressed air ( i had the problem of 8GB RAM installed and 3.99GB was usable and cleaning the slots solved it.) Still doesn't work? Get one RAM Stick and try it in all of the slots. if it doesn't boot up in one slot, the slot is damaged. also try all RAM Sticks one by one. if one doesn't work, the RAM is damaged. If your CPU heatsink is mounted too tight, try loosening it up a bit too

Your 16 GB of RAM is it 4x4 or 2x8 ?

Need System complete specs, especially the CPU, MOBO, RAM.

Do the following:
1. Check your motherboard manual to see how the paired slots are arranged. Usually they are 1+2 and 3+4, but some boards are weird and do 1+3 and 2+4, or even 1+4 and 2+3. You need to know which one you have.
2. Check which of your RAM sticks is paired with which, do NOT mix them up, and reinstall your double checked RAM pairs into their respective paired slots.
3. Did you buy a 16 GB kit or got 16 GB via 2 8 GB kits ? Sometimes RAM will just not work when its 2 kits, even if the same brand and model.
4. In your bios, you should be able to tell which one of your RAM slots is not reading, or not fully reading your RAM, make a mental or paper note of it.
5. Make sure your mobo actually supports the your RAM at the quantity, speeds or XMP you are trying to run it at. Sometimes boards will support ram, but not at all speeds or at all quantities / configurations.

Now do the following:
Reset BIOS RAM settings to defaults, power down.
Discharge your computer and remove all RAM. Clean them + the slots.
Get only 1 of the sticks, from a slot that was NOT missing/misreading any RAM.
Plug it into a verified good slot closest to the CPU. One of the slots that was NOT misreading, if the closest to the CPU slot is the one that was misreading, then go to the next closest.
Boot up your computer, see if it reads proper amount of off the stick you put in.
Rinse and repeat above for each RAM stick seperately.

This will tell you if any of your RAM is bad. If all your RAM checks out good and your board specs fully support the amount and speeds, then you have a bad slot. If one of your RAM sticks is bad, then you will need to replace the kit. I do not recommend mixing kits, especially on systems with known RAM issues.

If you have the option, for example your kit is 2x8 but you have 4 slots, or your kit is 4x4 and you have 6 or 8 slots, then obviously try the other slot pairs. See if that works. Otherwise, if it turns out to be a bad slot, your only other option is to get a new board.

Your RAM is not listed on the memory support list for that mobo http://www.gigabyte.com/products/product-page.aspx?pid=4717#memory%20support%20list

That doesn't mean it will not work, but is not guaranteed to do so. But 1st things 1st, do the above checks. If you end up switching your RAM I highly recommend going with a 2x8 kit from that list. Yes the 4x4 are slightly better performance wise, but the 2x8 are less headaches.

Double check that you did not RESERVE MEMORY for video use.

The "However, in windows and in task manager it says 16GB of ram but only 12 usable. " makes it seem that your PC hardware is fine, but that you have told windows to reserve the space.

Verify your PC HW is fine by going into the BIOS and displaying installed memory size. If you see all 16 GB then stop worrying about clean slots, etc. That is not your problem.

msconfig, boot tab, advanced options, uncheck the maximum memory if it is limited.

Run CPU-z (google it, download it) and goto the SPD tab, and then use the "memory slot selection" pulldown to see that each of the 4 dimms in your 4x4GB shows up.