We proposed a quantitative method for assessing chlorophyll (Chl) content and the Chl a/b ratio using a filter-free wavelength sensor. Initially, an experiment was conducted to simulate changes in Chl a and Chl b by adjusting the light intensity at wavelengths of 650 and 660 nm. The sensor’s output current ratio exhibited a linear relationship with the light intensity, suggesting its potential for measuring Chl. Subsequently, measurements were performed using standard Chl a and Chl b solutions. A strong linear correlation was observed between the sensor output and Chl content, independent of changes in the Chl a/b ratio. However, no direct correlation was found between the sensor output and the Chl a/b ratio. This inconsistency was attributed to the varying impact of Chl content changes on the sensor output, depending on the specific Chl a/b ratio. To address this limitation, we proposed a calibration method based on the relationship between Chl content and sensor output. By applying this calibration curve, we successfully reduced the error in Chl a/b ratio measurements. This approach enables the use of filter-free wavelength sensors for the accurate quantification of Chl content and Chl a/b ratio, offering a potential solution for on-site monitoring and agricultural applications.