ColleenA

To determine the volume of an RBC unit we use an empty bag, usually a transfer bag and place this on the scale and then tare it to 0.  We remove the empty bag from the scale and then place the RBC unit on the scale to determine the weight.  We generally feel that for transfusion purposes this is close enough to the real volume to let things go with the weight on a small label.  When measuring the volume actually transfused we rely on the infusion pump to see how much was truly infused.
For quality control purposes, there are conversions for weight to volume.  These are in the AABB technical manual.  For RBCs from WB with no additive solution the specific gravity is 1.063, for additive RBCs it is 1.060.  Just divide the wieght by the specific gravity to determine the volume in mL.
Hope that helps,
Colleen