When I was designing the measurement circuit used in my How to measure smartphone power usage posting, I needed to design a RC low pass filter to it. I could have picked an electronics book and formulas in it, but instead of manual calculation I used Google to find a tool for that on-line. There are quire many filter design tools on-line. I ended up using RC Low-pass Filter Design Tool from OKAWA Elecric Design. This RC Low-pass Filter Design Tool was useful and worked well.