Estimating r^2 from F ratio

From: skeptic8000 (iswear99_at_Hotmail.com)
Date: 03/26/05 

Date: Fri, 25 Mar 2005 23:11:15 EST

Is there a quick, relatively easy calculation that one can use to approximate an r squared value when all you are given is F ratio and the degree of freedoms (i.e. within and between subjects df's are known)? If so could you provide me with the formula and derivation?

Thanks