Before you can calculate how much fertilizer you need for your lawn, you need to know how big it is. First, you need to break your lawn down in manageable measurable sections. Printing an arial view of your house, from somewhere like Google maps, makes it easier to get the whole picture. You need to measure each area and record them. There are yard tools that you can use to figure this out, but an easier way to do it is to use an app that is designed to make measurements from maps. You need to use these calculations to come up with total square footage.