You may find yourself struggling to remember how much RAM you have at your disposal, or just wondering how much is being recognized by Ubuntu. This tutorial details ways to find out.
Open System Info.
This can be done either in the dash, or by going to the gear icon (top right), opening System Settings, and opening System Info.
Under the Ubuntu Logo and Version Number, it will state Memory 5.5GiB.
In this instance, 6GB is installed, but 5.5 is recognized because they are two 3GB modules (sticks). In some cases, there may be overlap such as this.