Why is Windows Refusing to Use All of the RAM Installed on a Computer?

There is nothing quite like the awesome feeling of finally being able to perform a major upgrade on your computer, but what do you do when your system refuses to make use of the whole upgrade? Today’s SuperUser Q&A post has the answer to a frustrated reader’s question.

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.

The Question

SuperUser reader Chairman Meow wants to know why his Windows 7 system is not using all of the RAM available on his computer:
I have Windows 7 Home Premium (64-bit) installed on my computer and the motherboard is able to handle up to 32 GB of RAM. I recently upgraded my computer to 20 GB of RAM, but the operating system says that only 16 GB of the 20 GB I installed is usable.
I have four slots on the motherboard. I installed two 8 GB sticks of RAM closest to the CPU and two 2 GB sticks of RAM in the remaining slots. I made sure that the RAM sticks are the same (DDR3, 1600 MHz). Just in case it matters, I also installed a GTX 770 GPU with 2 GB of memory. Here is a list of the specifications for my motherboard: P8P67_LE Motherboard (Asus).
What am I doing wrong? Why am I seeing this issue in my computer’s Control Panel?
Why is Chairman Meow’s Windows 7 system unable to use the other 4 GB of RAM?

The Answer

SuperUser contributor Canadian Luke has the answer for us:

Windows 7 Home Premium supports up to 16 GB of RAM. That is why only part of what you have installed is showing as usable even though your computer supports the extra RAM and does detect it. It is a licensing issue in Windows 7 that only allows it to use 16 GB of RAM. You can view the memory limits for various versions of Windows and Windows Server releases via the link below:
Physical Memory Limits for Windows and Windows Server Releases Note: This link is set to the Windows 7 portion of the support page.
Have something to add to the explanation? Sound off in the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.
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