Al, you said you guarantee your math, but you are wrong. A 5" and 6"
SQUARE are 25 and 36 square inches respectively, so your numbers are way
off. The numbers you came up with are for 10" and 12". The RADIUS of the
5" will be 2.5", and the 6 =3". We actually had a guy a work that said
two 9" ducts would equal one 18". That was about 5 years ago and we
still laugh about it. BTW, an easy way to figure area of a circle is
that it is about 21% less than that of a square the same size as the
diameter of the circle, whether it is 1 inch or 1 mile. Larry