Accurate results for primordial black holes from spectra with a distinguished scale

We perform an accurate computation of the production rate for primordial black holes (PBHs). The reason is that the underlying mass variance had been overestimated systematically, as was shown recently. For scale-free power law primordial spectra, and for a universe with critical density, the mass variance is less than 34% of its value thought earlier for the spectral index in the range 1less than or equal tonless than or equal to1.3. We then extend our study to spectra with a characteristic scale and find the accurate shape of the corresponding mass variance. For a pure step in the primordial spectrum, the step in the variance is smoothed around the characteristic scale k(s). For a spectrum with large oscillations near k(s), we find a pronounced bump in the variance. This could yield a significant part of the cold dark matter in the form of PBHs with a mass M in the range 5x10(15) gless than or similar toMless than or similar to10(21) g.