A factor of success in all grain brewing is how efficient your mash is. Most homebrewers can easily achieve about 70% mash efficiency. There are some fermentables that go into the boil, but are not expected to mash, such as sugar and honey.
If you need to know what the gravity is of your grain, you can refer to our Grain Gravity Value Chart.
Example : Grains Needed Calculator By O.G.
((original gravity - 1) X 5) / ((grain gravity - 1) X extract efficiency) = Pounds of Grain Needed
If your recipe contains more than one grain or extract, and many do, all fermentables in the boil kettle need to add up to the target gravity.