A test for determining spinning potential of cotton fibers on the rotor spinning system is proposed. It is based on the minimum twist required to rotor spin a fixed yarn size. Using known mathematical relationships inherent in the physics of yarn formation, the minimum twist results are transformed into the theoretically finest yarn count that may be spun commercially. The validity of these conclusions is empirically tested and verified on twenty different cotton samples. Implications about the effects of some key fiber properties on rotor spinning performance are clarified.