I can't remember where I learned this (among other things, but that's for another day) but it has been useful and fun.
To compute the weight of a lead round ball, take the diameter in thousandth of an inch, cube it, then multiply by 1502.6 to get its weight in grains.
Example: .450 X.450 X.450=.0091125 X 1502.6= 136.924, or effectively 137 grains.
Enjoy.
Three Hawks