Area from perimeter is it possible?

Hi

I want to work out the surface area of my lawn. Problem is, is that it is a very convoluted shape but there are no breaks in the perimeter. Now I'm not great at maths, I'm ok with length x width m2, and LxWxH m3, but I dont know how to find the surface area of circle or the volume of a cylinder.

I'll tell you how I think I could find the area of my lawn:

Using a meter wheel I walk around the complete perimeter of the lawn until I arrive back where I started.

Say it measures 100 meters.

If I get a piece of rope exactly 100 meters long, and then join the ends together and make a big circle, could I then just work out the area of that circle using pi r squared or whatever it is to get my answer? (I would have to learn how to do the Pi thing though)

Would this be possible do you think? or am I missing something?

I would have thought that the area of this rope circle couldn't change as it is bound by the 100 meter perimeter length of the rope. So even if I made a big letter 'C' on the floor with rope the area of that C would equal the circle or any other irregular shape I made.

I'd greatly appreciate anyones help..

Thanks