Check the output of
free -m. It will look similar to this:
total used free shared buffers cached
Mem: 7459 1483 5975 0 63 730
-/+ buffers/cache: 689 6769
Swap: 9536 0 9536
With different numbers probably. The first line ("Mem:") gives you the overall consumption, including cache and buffers (to speed up things, Linux uses free RAM to cache things like e.g. file system info -- see the "cache" column in my example. If the RAM occupied by this cache is needed elsewhere, that cache will shrink to free it). So look at the second line ("-/+ buffers/cache:"), which gives you the amount of memory really used by applications (plus kernel etc. of course). This should usually rawly match your calculation.