Schwarzschild radius formulae (i.e., formulae for the radius of the event horizon of a Schwarzschild black hole):

    1. General Schwarzschild radius formula R_sch = 2GM/c**2.

    2. Fiducial-value formula for stellar-mass black holes: 
      R_sch = 2.95318 km (M/M_☉) ≅ 3 km (M/M_☉)  .
    3. Fiducial-value formula for supermassive black holes (SMBHs): 
      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)]  .
    Even largest supermassive black hole known only have masses up to only of order 10**10 M_☉, and so can only have Schwarzschild radii of order hundreds of astronomical units.

    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.

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