What is the specific formula to calculate the area of an oval?

I ask a question because pi would be the ratio of a circle's circumference to its diameter, and wouldn't this number change if it is not a perfect circle and an irregular circle? I assume the formula would very for each oval you have. Wouldn't you just divide the circumference of the oval by the diameter of the oval and multiply it by the radius squared? Also, is there a way with modern stencils or utensils to create a certain oval? If so, mathematically, how can and is this done? Thanks.