Dell XPS 15 9550 32gb only 25.4gb usable

There is no way to free up all system memory - the operating system requires a certain amount to function.

@ejn63, that "usable" metric doesn't subtract the amount of memory the OS consumes.  I've checked 2 systems, one running Win10 1607 and the other running Win10 1709 (both with Hyper-V installed, incidentally) and I don't see that "usable" metric at all.

That's typically seen on 32-bit systems because some devices (like integrated GPUs) use system memory as video memory, and some other hardware devices even when they have their own memory need to reserve a block of address space from the system-wide pool to refer to their own memory -- which means that actual system memory can't use that address space even though capacity is technically available.  But on a 64-bit system, the address space pool is so large that this isn't an issue anymore.  Do you have any "unusual" peripherals connected to your system at the moment, such as an external GPU via Thunderbolt?  If so, do you still see this metric if you boot your system with no external peripherals attached?  Also, do you have a Samsung retail SSD with "Rapid Mode" enabled, which uses a block of system memory essentially as a huge cache?

Rapid Mode isn't supported for NVMe SSDs, so that's expected, but in that case it wouldn't account for a large chunk of memory missing.  I don't have any immediate suggestions left in that case, but I'll do a bit of research as I have spare time because I'm rather curious about this now.  And if you find the answer yourself, please report back.

Ok, so you completely removed the original memory and installed a matched pair of 2x16GB instead.  In that case, one way you can identify a bad module would be to remove one of them.  Boot the system and see if it shows 16GB of memory with all of it usable.  If so, replace that single module with the other one and see what happens.  You may find that one of them shows 16GB with only 9.4GB usable or something.

Just saw your new post with Resource Monitor.  I agree the GeForce GPU seems like the only possible culprit that could take that amount of memory, and I admit I don't have experience with other XPS systems that have 32GB of memory installed; my own XPS 15 only has 16 and I don't have this issue.  It's certainly possible that my understanding is incomplete on this, but from what I do know, I'm not sure why the GPU would cause this behavior.  First, the GPU definitely has physical memory dedicated to it; the GPU does not reserve actual system memory to use as video memory.  And second, even if it required a block of address space, that shouldn't cause a large hardware reservation like that.  Even if it needed to reserve address space below the 4GB barrier, which many GPUs do, the rest of the system's memory should still have no problem mapping to a higher address space, since the 64-bit architecture offers enough address space for some outrageous amount of memory.  And in that case, you wouldn't lose that amount of usable memory.

Like I said, I'll keep digging on this because I'm curious myself.

Quick question: Did the system ship with 32GB of RAM, or did you add more yourself?  Apparently one possible cause of memory being "Hardware Reserved" is when the system detects that at least part of it is bad.

Windows Resource monitor does show 6,7GB reserved for hardware

So if this is not the SSD then the Geforce GTX 960M is only thing left that could claim such an amount of memory for hardware. 

I have no unusual peripherals connected but I did switch the factory default SSD for a Samsung 960 EVO 1TB SSD M.2 but Samsung's Magician tool says that Rapid mode is not supported for this SSD.

Bear in mind this system has a hybrid GPU system - the Intel GPU is ALWAYS active, even when the nVidia GPU is being used - so it can indeed consume RAM -- and Windows since 7 delegates quite a bit to the GPU in all cases, so it is likely completely normal to see what you're seeing.

This isn't Windows 7, and I have two other systems that have an Intel GPU.  My Latitude 7480 running Win10 1607 says 16GB with 15.9 usable, and my XPS 15 9530 with a GeForce GPU in addition to integrated graphics running Win10 1709 shows 16GB with no separate note about usable memory at all.

Please re-read before posting.  I stated "windows since 7" - not Windows 7.

And the load on the GPU will depend on what's running at any given point in time -- and it WILL vary from system to system and situation to situation.

Strange, both DIMM's separately provided 15,8 GB usable with only one installed. As soon as I installed both again it went back to 25.4GB usable. If this was memory reserved for GPU then I would expect more then 200MB to be reserved with 16GB.

Based on some threads I read in the meantime, in that case it sounds like that matched set may not play well with your system.  This post of this particular thread in particular describes somewhat similar behavior: www.experts-exchange.com/.../Installed-memory-RAM-24GB-7-99-GB-usable.html

This one is also somewhat useful: superuser.com/.../why-is-all-my-extra-ram-marked-as-hardware-reserved-in-windows-7

Agree that the matched set is most likely is not compatible with the system, I will see if I can test that.