I would suggest measuring the outside length and width of the property. Then subtract the area of the house and driveway or whatever else you need to subtract.
How are you doing it?
My brother told me to break it up into complete square or rectangle shapes.
So grass area 1 Could be broken down to a rectangle & a small square.
Measure the 2 individually by Base x Height then afterwards just add the 2 answers & I'll have my complete grassy Area 1 Sq ft.
So basically Sides 1-4 is the rectangle. (Measure Length x Height = X)
Then the left over small square, Sides 5 & 6. (Measure Length x Height = Y)
Then add both answers (X + Y = Total Sq.Ft)
I felt like a dummy afterwards for thinking inside the box instead of outside.