Seeded batch desupersaturation experiments of paracetamol in water were monitored using in situ ATR-FTIR and in situ Raman spectroscopy. On the basis of the linear relationship between the measured signal and the concentrations of the involved components, in situ ATR-FTIR and Raman spectroscopy could be used quantitatively in a calibration-free manner to estimate the crystal growth rate of paracetamol. Using a population balance model and a nonlinear least-squares optimization routine, it was shown that by fitting the measured time-resolved ATR-FTIR spectra directly and by fitting the desupersaturation profiles obtained from these spectra combined with a previously developed calibration model, essentially the same growth rate parameters can be estimated. Very similar growth rate parameters could be obtained by fitting the measured time-resolved Raman spectra using the same population balance model. These results show that crystal growth rates can be estimated in a calibration-free manner using both in situ spectroscopic techniques, thereby avoiding lengthy calibration procedures.