R_sch = 2.95318 km (M/M_☉) ≅ 3 km (M/M_☉) .
R_sch = 0.0197408 AU [M/(M_☉**6)] ≅ 0.02 AU [M/(M_☉*10**6)] = 19.741 AU [M/(M_☉**9)] ≅ 20 AU [M/(M_☉*10**9)] = 9.57061*10**(-5) pc [M/(M_☉*10**9)] ≅ 10**(-4) pc [M/(M_☉*10**9)] .
Kerr black holes have somewhat different corresponding formulae for the radius of the event horizon: i.e., Kerr-Schwarzschild radius. However, the formulae above are probably good approximations for real black holes which are in general Kerr black holes, but NOT those with super fast black hole spin for which Kerr-Schwarzschild radius formulae are needed.