Schwarzschild radius rₛ = 2GM/c², which is 2.953 km per solar mass. Surface gravity falls as 1/M, so supermassive holes have gentle horizons and stellar ones savage ones. Tidal acceleration across a length L at radius r is 2GM·L/r³. Hawking temperature T = ħc³ / (8πGMkᵇ) ≈ 6.17×10⁻⁸ K per solar mass, and evaporation time scales as M³ — which is why stellar holes outlive the universe by orders of magnitude while tiny ones would have gone already.