Increasing the alkalinity of natural waters by dissolving natural or artificial minerals is a promising mitigation solution to sequester atmospheric CO2 and counteract water acidification. Here we address the carbon-capture efficiency of water alkalinization by deriving a mathematical factor - referred to as the alkalinization carbon capture efficiency (ACCE) - that quantifies the increase in dissolved inorganic carbon in the water as a result of variations in water alkalinity. We show that ACCE strongly depends on the water pH, with a sharp transition from minimum to maximum carbon-capture efficiency in a narrow interval of pH values. We also compare ACCE in freshwater and seawater, and discuss potential bounds for ACCE in the soil solution. Finally, we calculate ACCE for 156 lakes located in an acid-sensitive region, highlighting the great sensitivity of carbon-capture efficiency to the lake pH, and for the global surface ocean, revealing a latitudinal pattern of ACCE driven by differences in temperature and salinity.