ESDU 78003 gives a method of obtaining the attenuation across a frequency band by integrating the discrete frequency attenuation values given in ESDU 78002 over the frequency range. Allowance is made for spectrum shape and guidance is given on an atmospheric layering technique for use when atmospheric properties vary over the propagation path. A computer program@ ESDUpac A7803@ is provided. It calculates band attenuation using an ideal filter shape@ while a rapid graphical procedure for obtaining the one-third octave band attenuation is also included. The program gives band attenuation for up to 34@ constant percentage or constant bandwidth@ frequency bands. A maximum of 20 layers may be used to specify the variation of temperature and relative humidity along the propagation path. Two worked examples illustrate the use of the program and the format of the input and output. The program can also be used to add or subtract band attenuation from spectra input to the program using the Common Data File system.