Homemade colorimeters using simple parts are excellent in undergraduate teaching labs as they allow students to learn instrument components and principles behind absorbance measurements. However, the absorbance values measured from such instruments often differ from those determined by commercial spectrophotometers. This work provides a step-by-step calculation of absorbance, taking into account of the properties of each component. Using a Si-photodiode detector (PD) and a light-dependent resistor (LDR), we show how the absorbance measurements depend on the spectral irradiance of the light source, the absorption spectrum of the sample, and the responsivity of the detectors. The provided calculation can help undergraduate students visualize how these factors contribute to optical measurements. It can serve as an experiment to introduce concepts of instrumentation and analysis of its performance.