> Jives with my mental benchmark, 100K/0.1. I do the math in my head relative to that
Mine is 0.01u for 1Meg; augmented by 100u for 1k.
This is for -3dB at <20Hz, as I wrote. Somebody here wrote "15.9Hz" which is true but fruitless. Assume caps and resistors can be 30% off their markings. Bad day at the factory OR 20 years of heat.
And you want only about ONE "20Hz" cut in a complete system. Six 20Hz cuts cascaded is -3dB at 80Hz and even a guitarist might notice (and like it). It is not uncommon to find ten bass-cuts in one box. As has been said here, most of your couplings should be aimed 2Hz or below. If you have "20Hz" memorized (or written on the restroom wall), transposing to 2Hz "should" be easy. If not, maybe take up origami?
And I have always measured frequency response with a meter. And a proper signal generator (Sine). While hiss-testing has use in acoustics, it's just wrong for electronics. And always needs more equipment than a plain HP 200AB and 400E ACVM.
But the title of this thread is "Input Coupling Capacitor". It may be utterly reasonable to have the FIRST blocker inside the box up near 20Hz, so subsonics do not muddy-up the input stage. And that's the one where we *might* use a calculator. All those others, we would be figuring "2Hz here means > 1.2345uFd", look in the catalog, 10uFd is same-price as 1.5uFd, we use 10uFd and probably a lot of places. (Some consoles use 220uFd in "high" impedance, ~~10k, links; with maybe one input and one output coupling cut a little closer.)