Couldn't say it better, this is exactly right. For every two-part product that we sell, the mix ratio we provide is in part-by-weight, not parts-by-volume. Because the density of the materials (the density of the resin vs the density of the hardener) are different, this means that you can't use the parts-by-weight ratio as a parts-by-volume ratio. We chose parts by weight because it's generally a much more accurate way of measuring (for a number of reasons). The only exception across our whole range is ArtResin (which uses a 1:1 by volume mix ratio) or when measuring MEKP catalyst into polyester resin or vinylester resin in which case most people use a catalyst dispenser bottle to measure catalyst in by volume.