Water is fundamental to magma genesis, evolution, and eruption. Few direct measurements of magmatic H2O exist, however, because rocks found at the surface have extensively degassed upon eruption. Olivine-hosted melt inclusions provide a standard approach to measuring volatiles in undegassed magma, but many volcanic deposits do not contain melt inclusions large enough for analysis (>30 μm), or olivine at all. Here we use an AlIV-dependent partitioning relationship to calculate magmatic H2O from direct measurements of H2O in clinopyroxene phenocrysts. We test this approach using phenocrysts from four arc volcanoes (Galunggung, Irazú, Arenal, and Augustine) that span the global range in H2O contents as measured in olivine-hosted melt inclusions (from 0.1 to 7 wt% H2O). The average and maximum magmatic H2O contents calculated from the clinopyroxene measurements agree within 15% of the melt inclusion values for most of the samples. The evolutionary paths recorded in H2O-Mg# variations overlap in some clinopyroxene and olivine-hosted melt inclusion populations, and in others, the clinopyroxenes record a larger portion of the liquid line of descent or a different portion of the magma system. Thus, the use of phenocrysts to estimate magmatic H2O contents creates a new and powerful tool in igneous petrology and volcanology.