Often we need to know how much CO2 we can store in a certain underground space, or how much such space we need to store a given amount of CO2. In a recent attempt (Bradshaw et al., 2006) to list various regional and global estimates of CO2 storage capacity (Figure 1), the estimates reported are often quoted as “very large” with ranges in the order of 100 to 10 000 Gt of CO2. It is clear that there is a lack of definitions, rules, and general procedures for calculating storage potentials. The calculations of CO2 storage capacity are based either on the solubility potential (Bachu et al., 2003), which ignore volumetric consequences and the kinetics of this process, or on a fixed percentage of a subsurface volume (Koide et al., 1992). None of the cases using the latter approach include any definitions. In this paper, we will specify a method to calculate the theoretical maximum storage capacity. We will give comprehensive definitions of all the important parameters (here in italic) in a separate and upcoming publication.