On a planetary scale, the carbon cycle describes the movement of carbon between the atmosphere and the deep Earth. Carbon species are involved in diverse Earth processes, ranging from sedimentary, metamorphic and igneous petrology to the long-term viability of life at the Earth's surface. Volcanoes, and their associated magmatic systems, represent the interface through which carbon is transferred from the deep Earth to the surface. Thus, quantifying the CO2 budget of volcanic systems is necessary for understanding the deep carbon cycle and, concomitantly, the CO2 budget of the near surface, including the atmosphere. In this review, Kilauea volcano (Hawaii) is used as a case study to illustrate simple calculations that can account for processes that affect the amount and distribution of CO2 in this relatively well-studied volcanic system. These processes include methods to estimate the concentration of CO2 in a melt derived by partial melting of a source material, enrichment of CO2 in the melt during fractional crystallization, exsolution of CO2 from a fluid-saturated melt, trapping and post-entrapment modification of melt inclusions, and outgassing from the volcanic edifice. Our goal in this review is to provide straightforward example calculations that can be used to derive first-order estimates regarding processes that control the CO2 budgets of magmas and that can be incorporated into global carbon cycle models.