In experiments, strontium cobaltite (SrCoO3) has been confirmed to be a ferromagnetic metal (Curie temperature TC≈305K) at ambient conditions and remains in cubic perovskite structure up to ∼60GPa. Using local density approximation + self-consistent Hubbard U (LDA+Usc) calculations, we show that ferromagnetic metallic SrCoO3 at low pressure is in an intermediate-spin (IS) state with d6LÌ character: nearly trivalent (Co3+) instead of tetravalent cobalt (Co4+) accompanied by spin-down O-2p electron holes (ligand holes LÌ). Our calculations further predict that upon compression (â‰7GPa), SrCoO3 undergoes a transition to a low-spin (LS) ferromagnetic half-metal with an energy gap opened in the spin-up channel. Compared to the metallic IS state, the half-metallic LS state exhibits even more prominent d6LÌ character, including nearly nonmagnetic Co3+ and exceptionally large oxygen magnetic moments, which contribute most of the magnetization. By analyzing x-ray diffraction data of compressed single-crystal SrCoO3, we point out an anomalous volume reduction of ∼1%. This previously unnoticed volume anomaly is in great agreement with our predictive calculations, providing quantitative evidence for the simultaneous metal-half-metal and spin transition in SrCoO3.