This web site is in french, but the calculator really is a good tool for crafting beer correctly.
HELP FOR TRANSLATION: In the section "Brassage" (Brewing), you can see a green subtitle named "Un peu de pratique". Here is the calculator. The first case is the volume (in liters) you want and the one next to it is for the alcohol percentage you want (example, write down 5 for a 5% ABV beer).
Then numbers show up ; the one on the left show you the quantity of malt in kilograms you need (for both primary and secondary malt together). The center one shows you how much water you need to use in the brewing part, and the last one shows you how much water you need to use during the filter part (90°c water) on the grain.