Simple syrup is an extremely common ingredient in cocktails and, as the name implies, very simple to make: it’s just sugar and water in a 1:1 ratio. What’s slightly less easy is knowing how much water and sugar to measure out to get a given volume of syrup.
My calculator uses the densities of water (1g/ml) and crystalline sucrose (1.5862g/ml) to do a fairly naïve estimation of the final volume.
This isn’t totally accurate because densities don’t quite work that way in real life. Possibly the most well known example of this is what happens when you mix ethanol and water (250ml + 250ml = 480ml).
However, for cocktails this gets us in the ballpark and the small difference doesn’t really matter.
Recipe
Combine both ingredients in a saucepan and cook over medium heat until all the sugar has dissolved.
Transfer immediately to a heat-safe bottle or let it cool to room temperature in the saucepan and then transfer to a bottle.
Simple syrup should be refrigerated unless consumed within a day or so. The sugar content is too low to prevent the growth of microbes.