Rapid estimations of diameter minus lumen(D-L), and outer perimeter of cotton fibers (P) inmicrons, as a biological fineness of Egyptian cottoncould be calculated with satisfactory levels ofaccuracy from hair weight (H.W) in m/tex andmaturity ratio )MR) obtained from Micromat data(new F/MT instrument), using the following equations:2Circularity x 3.14 x 1.52Hs (Standard finenessDiameter (microns) or Final format (D) (microns) = 1.205 HsorCircularit y × 1.524 x 3.14 x HsPerimeter (microns) or Final format (P) (microns) = 3.7853 HsResults of the current study suggested thatmore attention should be focused on meaning andmeasurements of the three values of biologicalfineness (i.e.) diameter (D), perimeter (P) (microns)and standard fineness (Hs) m/tex. whichcan be derived from the data obtained from Micromatinstrument.