This protocol describes a standardized in vitro method to determine the digestibility and digestible indispensable amino acid score (DIAAS) of dietary proteins. This ‘INFOGEST Quant’ method is an extension of our previous INFOGEST static digestion model (INFOGEST 2.0) and adds a workflow for the quantification of total protein digestibility, individual amino acid digestibility and DIAAS. The protocol was validated using in vivo data obtained by digesting the same food samples, and the results showed a high degree of agreement, confirming its relevance for nutritional assessments. To establish the DIAAS of a protein source, nonabsorbable peptides and proteins are precipitated after in vitro digestion and the resulting absorbable fraction is analyzed using ultrahighperformance liquid chromatography with ultraviolet detection (to assess total and individual amino acids). Two alternative quantification strategies are also described: protein titration using the Kjeldahl method (to assess total nitrogen) and spectrophotometric analysis with o-phthalaldehyde (to assess total amino groups). Both alternative methods are valid only for the calculation of total digestibility and the proxy-digestible indispensable amino acid ratio, which gives an approximation of the DIAAS of the tested protein sources. Compared to existing approaches, this protocol is suitable for routine application in nutrition and food science laboratories. The preparatory steps take ~6 d, whereas the full workflow can be completed in triplicate in ~8 d. Analysis of the digesta takes an additional 3–5 d, depending on the method. The procedure requires only standard laboratory equipment and reagents and can be performed by anyone with basic training in biochemistry or a related discipline.