There is a misconception here. The memory usage graph does not account for all memory usage. It was never intended that it would. If you look at the Resource Monitor memory tab you will see a substantial amount of memory shown as "Standby". This memory is very much in use as a kind of cache and contributes substantially to good performance. All memory except that labeled as "Free" is in use. If Standby memory was included in the graph many systems would show near 100% usage most of the time and that would be neither interesting nor useful.
But with 18 GB RAM you must face the fact that much of the time you will have substantial amounts of RAM that is Free and unused. With a relatively light workload there is simply no way that such large amounts of memory can be put to efficient use at all times.
Microsoft has devoted an enormous amount of effort to the system memory manager to make it use whatever memory resources are available to the fullest possible extent. Within reasonable limits adding memory will improve performance. But there will always be a point of diminishing returns beyond which more memory does little to nothing. With 18 GB I expect you are well past that point. With typical workloads that limit is roughly 8 GB on a 64 bit OS. Of course there are exceptions, such as when running virtual machines or when the workload is very high.
The suggestion in post #4 is not useful in such cases and will accomplish the exact opposite of what you want. The purpose of the option is to restrict memory availability for testing and diagnostic purposes. It will in no way force the OS to make more efficient use of memory.
If you post the screenshots suggested in post #2 it will be possible to more fully explain the situation.
Edit: After seeing the screenshots it appears that all but 1926 MB of RAM is actually in use. With 18 GB RAM I would consider this very good. Any attempt at manipulating this is unlikely to be productive and may well impair performance.
