This is not as simple as it seems. If you are looking for accurate numbers you need to realize that ASME B16.9 requires that the thickness be not less than 12.5% less than nominal, and that the fitting must have a pressure rating equivalent to straight seamless pipe. The maximum wall thickness is not really specified. Technically (and somebody correct me if I am wrong) a manufacturer could produce all schedule 160 fittings, taper bore the ends to schedule 40, and mark/sell them as schedule 40 fittings. The internal profiles of fittings could vary widely amongst differnt manufacturers. For example, picture a tee manufactered by machinging a forged block, and then picture a tee manufactured by forming an extruded outlet. Both fittings could comply with B16.9, but the metal volumes, and the internal volumes could differ significantly.
Certified drawings from manufacturers might help. Otherwise, I like Latexman's suggestions for determining either the internal or metal volumes.