The errors caused by monochromator bandwidth in spectral responsivity measurements with a monochromator-based apparatus are discussed. Bandwidth effects are not negligible in high-accuracy cryogenic radiometer-based calibrations. A simple numerical method is used to calculate bandwidth effects for different types of detectors, monochromator slit scattering functions, and monochromator output spectral distributions. The method uses low-order Lagrange polynomials fitted segmentwise to measured spectral responsivity and monochromator spectral distribution data in order to make the calculations. It is shown that the shape of the slit function has only a small influence on the bandwidth errors, whereas the output spectral distribution of the monochromator can strongly affect bandwidth errors. It is also shown that in most cases the magnitude of bandwidth effects will vary as the square of the bandwidth. Bandwidth error calculations are presented for various types of detectors (silicon, silicon trap, germanium, InGaAs), for a V(lambda) detector, and for a typical filter radiometer. A comparison is made between calculated and measured bandwidth effects to validate the method used. In general, calculations of bandwidth effects will be mostly useful for determining uncertainties associated with monochromator bandwidth in spectral responsivity measurements; however, in certain cases the calculations can be used to apply corrections for such effects.