We all learned at school that percentages have no
units; however, when we derive a percentage for the
solvent or water content, we do so on a weight or
volume basis. For example, if we have 2 lb of solvent
in one gallon of coating that weighs 10 lb/gal, then
the percent solvent content by weight is 20% w/w.
For the same coating, if the volume of solvent in the
same gallon can is 0.30 gal, then the percent solvent
content by volume is 30% v/v.