The wikipedia article has all the charts and formulas you need. https://en.wikipedia.org/wiki/Alcohol_by_volume https://en.wikipedia.org/wiki/Alcohol_by_volume

The wikipedia article has all the charts and formulas you need. https://en.wikipedia.org/wiki/Alcohol_by_volume https://en.wikipedia.org/wiki/Alcohol_by_volume (To make the above solution 50% abv, btw, you need to keep adding water until it's 2 (liters, whatever) total volume.)