GHB generally comes either in pure powder form, or mixed into water. In powder form, measuring a dose isfairly straighforward. In liquid form, GHB comes in a wide variety of concentrations with a single dose ranging from a few drops to a full glass. If you don't know the concentration of your liquid GHB, be extremely careful. There is no way, without testing, to determine its strength. Don't assume anything. Carefully check the concentration with your source before calculating your dosage.
GHB is extremely dose sensitive. Each person has a different response to each dose level. To safely verify the correct dosage, ALWAYS START LOW.
http://www.erowid.org/chemicals/ghb/ghb_dose.shtml