As per our PMâ€™s, what you are doing is drawing an object and then trying to say it weighs 5kg ( which is OK but you canâ€™t expect the math to work in that way)â€¦The object needs to be scaled to fit its weight.

A solid disk of cast iron needs to be 159.5mm x 35mm to weight 5kg

if you draw up a cylinder with these dimensions and calculate the mass it will show 4.99kg in the solid body slotâ€¦

Like I said in the PMâ€¦If you want this object to fit a certain weight you are going to have to do ALL the calculationsâ€¦

*I am not sure if it could be done with a ratio or notâ€¦I havenâ€™t taken math for 40 yearsâ€¦*
Is where I got the size of a disk to find the radiusâ€¦I left 35mm thick and adjusted the diameter till I got 11.2lbs =5kg

Then in Blenderâ€¦made a cylinder to those dimensionsâ€¦

Select the disk and add the ridged bodyâ€¦ donâ€™t enter anything in the Mass

Bring up Calculate Mass-Cast Ironâ€¦and check the KG in the soft Bodyâ€¦4.99Kg

PROBLEM !! The object is a batch of different partsâ€¦you have to subtract all those volumes from the overall and still try to keep 5Kg ???

*( I only have half the math skills you would need to do that!! )*