low pass filters

I'm trying to figure out a formula that allows a low pass filter to pass a 100hz ?

You want to pass only below 100 Hz?
Or 100Hz aand above?
Your question could be interpreted either way.

Google low pass filter, there are lots of options:
actve, passive, sharpness of filter rolloff, etc.

Here's the simplest you can get

http://sim.okawa-denshi.jp/en/CRlowkeisan.htm

It would be best to try and restate your goal. If you are using a single specific frequency of say 100Hz then a bandpass filter may be best to use. If you wish to pass frequencies from 0 to 100Hz then a low pass filter with a cut-off frequency some value above 100Hz (how higher above depends on filter type and number of poles used) would be used.

Filters can be as complex or simple as one can imagine. It would be helpful if you could characterize your 100Hz signal, what is it's voltage, wave shape, and what impedance (source current drive) does the signal have to be able to provide to what it will be wire to?