Boron and its isotopes, ¹¹B and ¹⁰B, can track magmatic–hydrothermal processes, but this requires knowing how the isotopes fractionate between melt and fluid, which depends on B coordination. This is known for aqueous fluids but not for silicate melts. We determined B coordination by nuclear magnetic resonance in granitic glass with variable water content and alkali–alumina ratios. The results provide a way to estimate B coordination and melt–fluid fractionation for variable melt compositions.