Since you mentioned a two-turn circular loop I am presuming you are interested in a so-called "magnetic loop". I am not aware of any online calculators but the prevailing wisdom on these seems to center on the idea that the total wire length in the loop should be .085 wavelengths at the highest frequency of interest.

Since the top of the 80m band is 4 MHz, you have 637 cm (251 inches) of wire (or tubing).

Since the perimeter of a circle is 2*pi*R, this is a loop diameter of 80 inches.

A 2-turn loop would be half of that at 40 inches diameter.

You might check out the N6RK webpage for ideas on how to couple and feed the loop.

http://www.n6rk.com/loopantennas/pacificon.pdf- Steve