Nanosized carbon clusters self-formed in polymer-derived ceramics (named free carbon) play a key role in determining the properties of the materials. However, it is difficult to quantitatively measure the concentration of the free carbon. Lack of this information restricts the understanding of the properties of the materials. In this paper, we report a Raman spectroscopy-based quantitative method to measure the free carbon concentration by using silicon as an external reference. We demonstrate that the technique provides a simple and reliable way for the carbon content quantification.