A rule of thumb is one pound of liquid extract per gallon of water for a light bodied beer. One and a half pounds per gallon produces a richer, full bodied beer. A pound of LME typically yields a gravity of 1.034 - 38, as measured by a hydrometer, when dissolved in one gallon of water. DME yields about 1.040 - 43. These yield values are referred to as Points per Pound per Gallon. If someone tells you that a certain extract or malt's yield is 36 points, it means that when 1 pound is dissolved into 1 gallon of water, the gravity is 1.036. If that 1 pound is dissolved into 3 gallons, its gravity would be 36/3 = 12 or 1.012. The gravity is how the strength of a beer is described. Most commercial beers have an Original Gravity (OG) of 1.035 - 1.050.
Example of Gravity Calculations
If you want to brew 5 gallons of 1.040 gravity beer, this would call for 5 lbs of DME having 40 pts/lb./gal, or 5.5 lbs of LME having 36 pts/lb./gal.