How do I equally balance tmux(1) split panes?

I'm looking for a behavior that is similar to how vim(1) handles its split windows with ^w =. I know tmux(1) has predefined layouts with ^b Meta[1-5], but this likely does not have the layout that I am currently using. When splitting a window, it halves the current window for both panes. Split again, and it halves that pane into two new. Combine vertical and horizontal splits, and they continue to