According to Anthony C. Robin, an approximate formula for the capacity of a sealed expanded bag is:
where w is the width of the bag (the shorter dimension), h is the height (the longer dimension), and V is the maximum volume.
A very rough approximation to the capacity of a bag that is open at one edge is:
(This latter formula assumes that the corners at the bottom of the bag are linked by a single edge, and that the base of the bag is not a more complex shape such as a lens).
or roughly 0.19. According to Andrew Kepert at the University of Newcastle, Australia, the upper bound for this version of the teabag problem is 0.217+, and he has made a construction that appears to give a volume of 0.2055+.
In the article referred to above A C Robin also found a more complicated formula for the general paper bag. Whilst this is beyond the scope of a general work, it is of interest to note that for the tea bag case this formula gives 0.2017, unfortunately not within the bounds given by Kepert, but significantly nearer.