Anyone can calculate the area of a simple polygon like a triangle. But that's not what life usually throws at us. Typically the polygon is the outline of a room with odd angles or a plot of land that's anything but square. You can always "guesstimate" the size of the area but that can lead to paying more for materials than necessary and if we're talking about nice wood flooring for example, you could easily over-spend by hundreds of dollars.
With the Surveyors Formula, you simply take your measurements, transfer them proportionally onto a piece of graph paper and record the X and Y coordinates at each vertices and key these into the appropriate input fields on the app then press calc. It sounds a little tricky but it's really not. Read the app description in the app marketplace for detailed instructions.
The $1.29 you spend on this app will likely be the best "bang for the buck" you'll get all year.