Even in idealized, clear sky conditions, spectral UV models need well-specified input parameters such as solar zenith angle, pressure, ozone and aerosol optical depth to produce realistic results. UV models based on exact analytical methods are widely considered to be absolutely accurate with errors in results widely attributed to inaccurate input parameters. The uncertainties in the standard determinations of each input parameter were determined for the following methods of specifying each parameter: (1) ozone from satellites or ground-based Dobson spectrometer measurements; (2) aerosol optical depth from sunphotometer measurements; (3) single-scattering albedo; three different procedures using ground-based spectrometer measurements and radiative transfer calculations were studied; (4) ground albedo; the determination of the ground albedo is achieved using satellite measurements, which have a resolution of some 100 km; ground-based measurements may give some information about the albedo of the ground, but the albedo needed for radiative transfer calculations is a weighted average of the surrounding albedos. The resulting inaccuracies in UV calculations from errors in input parameters were simulated with the discrete ordinate method model by Stamnes et al. <1988> using the uncertainties specified above for the different input parameters. The simulations of the errors of the spectral UV calculations were carried out for each input parameter at 0¿, 50¿, and 80¿ zenith angle and analyzed at 305 and 380 nm. The overall maximum error may be determined by combining the different errors.¿ 1997 American Geophysical Union