I've just worked it out. I figure the people on this forum will be the only people in the world who actually have any interest at all in what the internal volume (capacity) of a double bass is. I calculate it as 94,060 cubic centimetres. Give or take a little. (area of top = 4,703cm2 x depth of 20cm). This is for a ¾ bass, 1920s or 30s German, flat back, gamba style bass. Maybe this will be of use to someone, sometime.