The amount of water you add really depends upon how soft or firm you prefer your pillow.
We recommend starting with 3 liters/quarts which would be classified as medium fill level. After sleeping on the pillow for a few nights you will know if this is the perfect amount for you, if not we recommend adding or replacing 1 cup of water at a time and testing the pillow while you sleep.
Soft= 2 Liters/Quarts
Medium = 3 Liters/Quarts
Firm = 4-5 Liters/Quarts
Maximum capacity is 5.5 Liters.