A semiempirical approximation to the extinction efficiency based on a modification to the anomalous diffraction formula is given and compared to the exact Mie computation. This approximation has been verified for complex refractive indices m = n-ikappa, where 1.01 </= n </= 2.00 and 0 </= kappa </= 10. The approximation is uniformly valid over all size parameters and has the correct Rayleigh and large particle asymptotic behavior. The accuracy of this formula is discussed as well as its computational advantages. The formula is also applied to some of the LOWTRAN aerosol models.