Absorption cross-section (ACS) of silicon nanocrystals (SiNCs) is determined via two completely independent approaches: (i) Excitation-intensity-dependent photoluminescence (PL) kinetics under modulated (long square pulses) pumping, (ii) absorbance measured by the photothermal deflection spectroscopy combined with morphology information obtained by the high-resolution transmission electron microscopy. This unique comparison reveals consistent ACS values around 10-15 cm2 for violet excitation of SiNCs of about 3-5 nm in diameter and this value is comparable to most of direct band-gap semiconductor nanocrystals, however, it decreases steeply towards longer wavelengths. Moreover, we analyze the PL-modulation technique in details and propose an improved experimental procedure which enables simpler implementation of this method to determine ACS of various (nano)materials in both solid and liquid state.