16GB installed (7.99 usable)

Check this http://www.sevenforums.com/tutorials/66482-memory-set-maximum-amount-used-windows-7-a.html

Did you confirm all sticks and slots work? What mobo do you have? Is this 2x8 or 4x4?

Are you using a dual channel memory kit or 2 single channel module?

Do you have them in the proper slots if using dual channel? refer to your owners manual.

Are you using a video card?

Usually dual channel is a kit of two modules of memory of the same size, i.e. 8GB + 8GB = 16 GB, that are placed in the proper slots on your motherboard per your owners manual. Dual channel actually works with CPU to run faster. It increases the bandwidth of transfer of information. Now, If you are using two single modules of the same size and were not part of a kit then they will operate as a single module only.

Check your packaging of the memory, it should say it on the label.

Be careful if it is dual channel. Make sure you place them in the right slots so it can utilize that feature. I learned from using ASUS boards to go right to left. Place one in the first slot then skip and place in the third slot. Figured that out after few hours trouble shooting an issue I had. Read your owners manual or check the makers website about it.

Could be what is causing the issue of how much RAM it is seeing.

Hope this helped.

Try updating bios and seeing what cpuz says is in every slot.

If you mean the windows memory diagnostic then that is mostly useless. You can try memtest but I doubt it will come up with anything.

@ivan, He already stated he is using 4x4. Channels wouldn't make the ram recognized but unusable.

Here is your answer, from the owners manual for your motherboard, page 2-7, note last sentence:

DDR3 memory modules are not interchangeable with DDR2 and the DDR3 standard
is not backwards compatible. You should always install DDR3 memory modules in
the DDR3 DIMM slots.

In Dual-Channel mode, make sure that you install memory modules of the same type
and density in different channel DIMM slots.

To enable successful system boot-up (Lynnfield CPU especially), always insert the
memory modules into the DIMM1 first.

Due to the chipset resource deployment, the system density will only be detected up
to 15+GB (not full 16GB) when each DIMM is installed with a 4GB memory module.

Forgot to mention, Windows 7 Home Premium will only recognize up to 16GB and the P55 chipset may see less also. Have Intel P55 chipset mother too.

On some boards it would cause the RAM to run as single channel only.

I joined just to thank this solution. I am experiencing this in win 10, here in the future.. and am using a p55 chipset.

I keep getting this (7.95gb usable of 16gb, single channel mode) every so often when overclocking, tried everything from pulling one chip and rebooting to voltages to timings, nothing worked, but then I tried down clocking the sticks (from 2133 to 1866) and unlinked the timings (and cloned the timing for the second chip) and rebooted and the ram was in dual channel mode again.

Rebooted and set back to max speed and it's still in dual channel mode (all 16gb usable). Hope this helps.

I have an msi 990fx mobo, amd fx8350@4.44ghz and 16gb gskill 2133mhz ram w. 9/11/11/31/6/42 timings.

I've done some testing and experimentation regarding this. First and foremost I had to do a resent clean reinstall of Win7U/64/sp1 and after fully updated (sans Win10 adware and self-installers). I had 32GB (4x8GB) installed in a dual-channel board. Type of RAM and MB is irrelevant at this point.

After having to transfer 2 of my DIMMs into a testrig setup I finally noticed a discrepancy in my available RAM % that seemed odd; looked at my System Properties, and there it was... "16GB installed (7.99GB usable)" and pondered briefly why this was there. Knowing full well that Win7HP/64 only allows up to 16GB of RAM and half my 32GB would've been usable; I opted for Win7U/64 instead to remove that limitation. So I knew that wasn't the issue, but checked my version to make sure. Yup, not the issue.

I then gave Google a shot... sure enough this is the first thread on the topic; old as it may be. But nothing here helped due to different components involved. Nor has the OP returned to confirm whether or not the only component-specific solution helped or was even noticed by more than other Google search passers-by.

From here I re-repurposed my 2 transferred DIMMs back into my main system for testing. At this point System Properties claimed 32GB Installed and 23.99GB usable. So now I knew it was a persistent 8GB allocated to 'something' for unknown reasons. It had to be boot-related. Whether it was boot.mgr or POST related remained to be determined.

So I decided to clip both possibilities in one go. I shutdown, removed 3 DIMMs so I would only have the 8GB available that was being allocated, Booted into the UEFI, disabled SVM (Secure Virtual Machine), saved and restarted into the OS. Checked System Properties and none of the 8GB was being allocated. Shutdown and reinstalled the 3 DIMMs and rebooted back to my OS. Checked System Properties again and none of the 32GB was being allocated.

So while some of the above may help on a case-by-case basis; I'm sure this will be the solution to other instances. Thank you all for your time and hope I've helped extend the helpfulness of this thread.

I'm going to close this since it's going on 3 years old. There's many reasons for not being able to use it all from windows versions, msconfig, memory remapping, dual sided compatibility, to bent pins and other issues. While this could really happen on any hardware, some causes will not. If you have an issue, make a new thread with your specs. But we don't need a long thread bumped and hijacked.