Colloidal transition metal nanocrystals have gained increased interest in several fields because of the fascinating structural, optical, and catalytic properties obtained at the nanoscale. A thorough and systematic understanding of their characteristics could prove beneficial for different applications. Among these characteristics, the optical properties of colloidal nanoparticles are especially of interest due to the intriguing features that emerge at sub-wavelength scales, such as surface plasmon resonance. However, most research in this domain has focused on metals with absorption maxima in the visible light range, while the optical characteristics of other species remain relatively less studied. Therefore, in this work, we report on the optical properties of uniform platinum, palladium, and nickel nanocrystals, which are widely used in fields from sensors to catalysis, yet, with regard to their optical properties, remain largely uninvestigated. In particular, we measure and analyze their extinction coefficients, showing a size-dependence that can be rationalized with simple equations that relate light absorption to the diameter of the particles. The comprehension of this relationship enables a simple, time-efficient, and accurate method to measure the concentrations of metallic nanoparticles in solution with UV-vis spectroscopy, a useful property which facilitates the preparation of materials based on these colloidal nanoparticles for further research.