Mix a bag of 25KG self-leveling concrete with 6L of water?
Lacking a specific brand from your question, I went and looked at 

http://www.screwfix.com/p/mapei-ultraplan-self-levelling-compound-grey-25kg/4959f

Which states that it will cover (mixed) 6 square meters at 3mm thick.

I make that 100cm x 600 cm x 0.3 cm or 18,000 cubic centimeters. Last I checked, 1000 CCs was a liter, so you have 6 liters of free space.