If I have a $1 \text{km} $ x $1 \text{km} $ plain of grass, then the surface area is $1 \text{km}{^2} $.
Well, no, because the Earth isn't a flat plane but rather a sphere that's curved. The surface area will be a tiny bit bigger but by just how much exactly? Is there a formula to find the surface area of a portion of a sphere's surface?
PS: I already know the Earth is oblate. For the purpose of this question just assume it's a perfect sphere of radius $6371 \text{km}$ with no imperfections (mountains, valleys, etc).