Dry malt extract usually contributes 45 gravity points per pound per gallon. This means that one pound of dry extract in one gallon with have a gravity of 1.045. Likewise, liquid extract will add 36 points per pound per gallon. Grain malt is more complex as each grain will add a slightly different contribution, and brewhouse efficiency in extracting sugars from the mash also affects the calculation; as a rule of thumb, you can figure that base malt and caramel/crystal malts will add 26 points per pound per gallon.
If, as an example, you have an all grain recipe with a 1.050 original gravity and it calls for 9 pounds of Pilsner malt, you could brew the extract version of that recipe by either using 7 pounds of liquid pilsner extract or 5.5 pounds of dry pilsner extract. Any of those recipes would give you a similar beer with a 1.050 original gravity.