Dry malt extract (DME) produces about 1.044 Specific gravity, per pound per gallon of water and liquid malt extract (LME) produces about 1.036.
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.
To determine how much malt extract to use to reach a predetermined starting gravity, use this simple formula:
degrees of extract) x pounds of malt used divided by gallons of beer to be brewed.
For example, if we use 6 pounds of liquid malt extract to make 5 gallons of beer, we would have 36 x 6 = 210... so, 210 divided by 5 (gallons of beer to be brewed) = 42, or a starting gravity of 1.042.