Finding the area inside a square

I want to be able to find the space inside a box basically, however the box isn’t always perfectly square. So my idea is to use worldspace on the boxes and then find the area using X,Y, and Z all seperately to find the positions, but im not sure if thats the best way or not. I’m not asking anyone to write it for me I would like to try it myself but if someone can pop out and tell me theres a handy function that does all that automatically that would also be helpful. thank you