Subsurface wells are widely used for a number of applications including groundwater supply, managed aquifer recharge, oil recovery, and carbon sequestration that need extraction and/or injection of fluids. One of the primary requirements for successful and cost-effective well operation at a given site is that the hydrogeological conditions must allow for a sufficient well extraction or injection capacity. However, little discussion exists in the literature regarding how to estimate these capacities and what the dominant controlling factors are. We developed methods for estimating the capacity of single and multiple wells both in confined and unconfined aquifers under constraints on hydraulic head, and performed a parametric study to estimate the influences of the parameters (transmissivity, storativity, well operation duration and well radius) on the extraction and injection capacities. For a single well, the parametric study showed that transmissivity has the most influence on the well capacity while storativity has the least. For multiple wells, we analyzed the impact of well interference on the total capacity of the wells for different geometrical arrangements of the wells (linear, circular and grid arrangements) and as a function of well spacing. The results showed that well interference can imply a significant reduction of the total capacity. More precisely, the total capacity increases much less than proportionally with the number of wells, up to reaching an asymptotic value (i.e., beyond a certain number of wells, increasing the number of wells yields a negligible increase in the total capacity). The results also showed that a linear arrangement of the wells yields a larger total capacity than a circular or grid arrangement. The results of this study provide fundamental insights into well hydraulics under hydraulic head constraints and have practical implications on site selection and well field design.