How do you calculate vertual memory?

Virtual memory is not calculated, although the operating system can manage its size based on how much memory you have. This can lead to fragmentation issues and slack space being created on the hard drive. A good virtual swap file size is generally your memory size multiplied by 1.5. Say you have 4G memory, the swap should be a fixed size (not allowing Windows to manage it) of around 6G. This will give it ample room to swap out information and stop it from fragmenting drive space.