Gravitational-wave (GW) and electromagnetic (EM) signals from the merger of a
Neutron Star (NS) and a Black Hole (BH) are a highly anticipated discovery in
extreme gravity, nuclear-, and astrophysics. We develop a simple formula that
distinguishes between merger outcomes and predicts the post-merger remnant
mass, validated with 75 simulations. Our formula improves on existing results
by describing critical unexplored regimes: comparable masses and higher BH
spins. These are important to differentiate NSNS from NSBH mergers, and to
infer source physics from EM signals.