Select a brew strength. Starting with "Regular" is probably your best bet.
If you're looking to brew a specific amount of coffee, start here.
If you're planning on using a specific amount of coffee, start here.
There a couple ways to use this calculator. A good starting point is selecting your brew strength an then entering the amount of coffee you want to brew in the water section. But if you're working with a specific amount of coffee then enter the amount of coffee you have left/want to use. After you've entered in either value the other will update based on the "Brew Strength" you selected.