This article is from the Beer and Other Brewing FAQ, by Kurt Swanson firstname.lastname@example.org with numerous contributions by others.
All fermentables (malt extract syrup, dry malt extract, grain malt,
sugar, honey, etc.) cause an increase in the specific gravity of the
solution when added to water. A common way to measure how much the
specific gravity increases is the number of SG points of increase
when a pound of the ingredient is added to one gallon of water.
Most fermentables used for beer are in the range of 25-45 points
per pound per gallon. Values for many of these ingredients may be
found in the references mentioned in the Bibliography section. When
substituting one fermentable for another, use the ratio of the
specific gravity contributions of each ingredient to scale the one
you will use to the amount that will provide the desired SG contribution.
Example: You have an all-grain recipe that calls for 8# of Malted
Barley, and you want to replace it with extract syrup. One of my
references lists the SG contributions of these ingredients as
approximately 30 points for the grain and 36 points for the syrup
per pound of ingredient per gallon of water. You multiply the
8# of grain in the recipe by 30/36 to get 6 2/3 pounds of malt