Today I built a water intake calculator. The recommended amount of water per day depends on your body weight and the amount of activity you do. You should drink your weight in pounds multiplied by 2/3rds + 12oz per 30 minutes of activity.
This site uses the stopwatch as a base for the CSS. I used this site to determine the math involved: https://www.slenderkitchen.com/article/how-to-calculate-how-much-water-you-should-drink-a-day. You can use this calculator here: https://zidri.github.io/WaterIntakeCalc/ or look at the source code on my GitHub.