How it works
f_c = \frac{1}{2 \pi R C} \qquad f_0 = \frac{1}{2 \pi \sqrt{L C}}
The same RC pair gives a low-pass (C to ground) or high-pass (R to ground) filter with the same cutoff. At fc the output is −3 dB (70.7 %); above it a first-order low-pass falls 20 dB per decade.
The time constant τ = RC sets the step response: 63 % after τ, 10–90 % rise in 2.2 τ, and within 1 % after 4.6 τ. That is the number to check for debouncing or an ADC input settling before a conversion.
When solving for a component, the nearest standard value is shown: E24 for resistors, E12 for capacitors.
Anti-aliasing before an ADC
Put fc well below half the sample rate, and size R so the ADC’s sampling capacitor can still charge through it (see the ADC’s maximum source impedance for your sampling time).