Unless you are a seasoned director of photography or assistant cameraman, chances are you can’t calculate f-stops in your head.  Fortunately there is a simple calculator available online that allows you to enter the starting f-stop and the number of stops to add or subtract and it figures the correct f-stop.

Also, if you need to determine the depth of field at a certain f-stop, you can use the DOF calculator.

Click here for the F-stop Calculator and here for the DOF Calculator.