Numerical simulations of volcanic processes show that the preeruptive and eruptive dynamics depend substantially on the amount and composition of volatiles present in magma, either dissolved in the liquid or separated in a gas phase. Although the investigation of melt inclusions within crystals is providing increasingly abundant data on the amount of volatiles dissolved in magmatic liquids, classic approaches involving phase relationships and experimental petrology cannot provide the relative mass of gas and liquid phases in magma. This represents a serious limit to the simulation of the dynamics of magmatic and volcanic processes. Here, a theoretical framework based on mass balance equations for the volatile species water and carbon dioxide in crystallizing, degassing magmas is developed. The theory presented offers a systematic description of the mass relationships between dissolved and total volatiles in magma, and leads to the definition of the total volatile (TV) line concept. The TV line represents the locus of total volatile contents consistent with the amounts dissolved in the liquid and measured in melt inclusions. Appropriate expressions for TV lines for closed and open system conditions with respect to the gas phase, and for crystallizing and noncrystallizing magmas, are developed. The TV line approach allows discrimination between melt inclusions formed in closed versus open system conditions and determination of total volatile budgets for magmas evolving under closed system conditions, and it can constrain the degassing history of magmas evolving in open system conditions. Several examples of the use of the TV line approach to determine total H2O and CO2 budgets in evolving magmas starting from melt inclusion data are provided. Applications to real cases suggest that total (dissolved plus exsolved) carbon dioxide contents in magmas can be in some cases from 1 to more than 2 orders of magnitude greater than the dissolved amounts.