How to count Tiles in a sourinding Area around an Object?

I want a object ,that is locked to grid ,to count all tiles, called "field",surounding it with a radius of 3 and than add to a public float called "grain"... roughly like this:

 void Update()
 {
 waitInt++;
 if(waitInt==100)
 {
 waitInt=0;
 grain += countedFields;
 }
 }

How can I get countedFields?