Traditional density functional theory (DFT) miserably fails to reproduce the experimental volume and magnetic anisotropy of D022 Mn3Ga, which has recently become one of the most sought-after materials in order to achieve a stable spin switching at low current density. Despite great progress over the last 10 years, this issue has hitherto remained unsolved. Here, taking into account the effects of strong electronic correlations beyond what is included in standard DFT, we show by comparison with the experiment that the DFT+U method is capable of quantitatively describing the volume and the magnetic anisotropy energy (MAE) in this alloy with physically meaningful choice of onsite Coulomb-U parameter. For the first time using a plane-wave code, we decompose MAE into spin channel-resolved components in order to determine spin-flip and spin-conserving contributions. The Mn atom at the tetrahedral site is identified as the primary source of the high perpendicular MAE with the most dominant spin-orbit coupling (SOC) occurring between its two orbital pairs: ↑↑ coupling and ↓↓ coupling between [Formula: see text] and d xy , and ↑↓ coupling between d yz and [Formula: see text]. Using the SOC-perturbation theory model, we provide interpretation of our numerical results. These results are important for quantitative microscopic understanding of the large perpendicular MAE observed in this material, and should assist in harnessing its potential for applications in futuristic spintronic devices.