rake paid fluctuates as a function of win rate and play style.
so you cannot calculate this exactly without knowing how many hands you've played in a stake and how much rake you've paid (in that stake). after which it's just simple math to work out your rake/100 and convert it to bb/100
i personally have paid 5.5bb/100 in rake at 10z pokerstars with 19 VPIP, not including rakeback. the rake there is 4.5% capped to $1.5 which is essentially uncapped until you have >150bb on the table.
As TB said above, you kind of just have to play a bunch of hands and then look at the stats with your tracker (HEM and PT both keep track of rake paid).
Most sites have a webpage somewhere showing what the rake is. It often works out at about 5% of the pot in online cashgames, and by coincidence that tends to work out at about 5bb/100 in both 6-max and full ring. Similarly, a 10% pot rake will usually average out at about 10bb/100.
If you play looser or more aggro (like in heads up or PLO), then the number can exceed 10bb/100 or even 15bb/100, but winrates in those games can also be pretty high if you have a significant skill edge, so it all cancels out, and most regs will be around or just above breakeven (post-rake) in whichever game they play.
