A 50kg bag of cement is generally 33l.
Generally 2 x 50kg bags fill 1 x 65l wheelbarrow.
If you are unsure, read the back of the bag. Or check with your manufacturer.
The RD is a comparison with the same volume of water at a controlled temperature. This should not be used to calculate the volume of cement in a bag.
Use the Bulk Density of that type of cement. That info can be obtained from the manufacturer.
An example is Ordinary Portland Cement (OPC) has a bulk density of 1500kg/m3.
This means it is 1.5kg/l.
50kg/1.5 = 33.3l.