How to get MDI client area minus control bars?

I wonder if there is an easy way to get the client area of an MDI application minus the control bars. What I mean is the total area for displaying child windows; control bars will block child windows. I can use GetClientRect() to get the whole client area of the application window, but then I would have to recalculate the resulting rect based on what bars are visible on the screen. There must be an easy way to do this.

thanks,

jonas

jonas