Why is the frequency measured in radians in filter design?

This is my first time posting a question, so sorry if my articulation is not proper.

I'v just started out with FIR filter design and I see the domain being -$$\pi$$ to $$\pi$$. Now I understand that discrete frequencies repeat after an interval of 2$$\pi$$ but consider this. I need to design a low pass FIR filter with a cutoff frequency of 100$$Hz$$. How can I specify this in terms of radians? Is it using $$\omega$$=$$2\pif$$? But that would be outside the domain. As you can see, I am very confused about this concept so any help would be appreciated.

The value of $$2\pi$$ radians corresponds to the sampling frequency. So the normalized frequency in radians is given by
$$\omega=\frac{2\pi f}{f_s}\tag{1}$$
where $$f$$ is the actual frequency in Hertz, and $$f_s$$ is the sampling frequency in Hertz.