I’ll give you the empiracle formula for working out the area of a circle.
From that you can figure out the area of any size Hula hoop you have and then
you can calculate your multiplication factor.
Area of a Circle in square ft = 3.14 x ( Radius of the Hula Hoop in Inches ) Squared
144 sq inches
EXAMPLE = If you have a 24 inch hula hoop which means it is12 inches in radius
The Area in sq ft of your hoop = 3.14 x ( 12 x 12 ) = 3.14
144 sq in
The sq feet in an acre is 43, 560. So to equal the square feet in an acre, given that there are 3.14 square feet in your 24 inch diameter hoop, you would need to multiply by

43,560 sq ft in an acre / 3.14 sq ft in your hoop = 13,872.

If you have a 27 inch diameter hoop you’d do this :

Your Area in sq ft = 3.14 x ( 13.5 x 13.5 ) Your Multuplication Factor for this diameter of hoop
144 sq inches would then be : 43, 560 / 3.97 = 10,972

= 572.26
144

= 3.97 sq ft


In the 24 inch hoop below there are 19 beans.The multiplication factor is 13,872 so there would be 19 x 13,872 = 263,568 beans per acre if every hoop thrown gave this kind of reading.
You need to do a minimum of 10 readings to give you any kind of reasonable average.