Simple syrup or rich syrup? Why 2:1 is not twice as sweet as 1:1

Simple syrup is one part sugar to one part water; rich syrup is two to one. Recipes just say "syrup", so which do you use?

Rich syrup is not twice as sweet: it is roughly 1.35 to 1.45 times sweeter, with less water added to the glass. Use the calculator to scale a batch by weight, see the °Brix and yield, and swap one syrup for the other without throwing the drink off balance.