Significance Carbon is critical to life on Earth, climate regulation, and many geological processes. Despite its importance, the amount of carbon in the planet has been poorly understood due to uncertainty in the composition of Earth’s core, likely the largest reservoir of carbon. Here, we demonstrate that carbon becomes increasingly compatible in silicate compared to metal at the high pressures and temperatures at which Earth’s core formed. Therefore, carbon can only be present in the core in low abundances, although the core still likely holds the vast majority of the planet’s carbon.