The semiempirical equation which was shown by Weissberg to be a very close approximation to the rigorous variational calculation of the pressure drop for creeping flow of incompressible viscous fluids through short tubes has been generalized to take into account the slip of a rarefied gas at the tube wall. The generalized equation is&ohgr;=&ohgr;s+&bgr;a/&lgr;¯, whereais the tube radius,&lgr;¯=(&mgr;/P¯)(12&pgr;RT/M)12, and&ohgr;=F/[14&pgr;a2&Dgr;P(8RT/&pgr;M)12]. Here, &mgr; andMare the viscosity and molecular weight of the gas, &Dgr;Pis the pressure dropP1−P2,P¯is the average pressure ½(P1+P2),Ris the gas constant per mole,Tis the absolute temperature, andF=P¯QwhereQis the volume flow. The coefficient &bgr; and the slip parameterwsdepend only onL/a, the length to radius ratio of the tube:&bgr;=(&pgr;/8)/[(L/a)+(3&pgr;/8)], &ohgr;s=(&bgr;2/3&pgr;)[(128L/a)+(27&pgr;2/4)]. Excellent agreement is noted between values ofwcalculated from the equation given here and values obtained by Knudsen for long tubes and by Lund for short tubes.