Origins
Ages ago, when I was first starting to mess around with electronics again, I found myself calculating RC filters. Constantly. They're one of those fundamental EE building blocks that find their way into everything, especially audio circuits and guitar effects. The math is simple, but more complex than I can do in my head.
There's quite a few sites out there that will do the math for you, but they tend to have weird and annoying interfaces. When I'm designing something or reading existing schematics, I want as little friction as possible, and often find myself plugging in all sorts of values to figure out cutoff frequencies or the right value to achieve a given cutoff.
So one evening, I hastily wrote my own. It lived on the now defunct glitch.com, and I happily used it (bugs and all) for many years.
Why DIY?
I'm a big believer in building your own tools, and this one has paid huge dividends over the years! It feels incredible to use something that works exactly the way you want it to, without suffering any small indignities in user experience. I highly recommend it, even if it feels like a waste of time.
Most tools have units as a separate dropdown, or omitted entirely. I don't think solely in Ohms and Farads, and am often interpreting existing schematics or have values in mind in various ranges. The path of least resistance (do not acknowledge this pun) is simply typing the units in; I can input "10k" and the conversion will happen transparently behind the scenes.
I wanted a tool that took inputs and gave results in the same format I think about these components, which almost always involves units. For example, if I need a specific cutoff and resistance it's nice to receive a rounded capacitor value in nano or micro Farads as a result. Less mental overhead doing unit conversions, more time spent solving the problem I intended. These little things add up when you're iterating over and over on something.
Sometimes, though, you want exact values, and you can now turn off this rounding logic and get the pure value with a disgusting degree of precision.
Revisiting
With the shutdown of Glitch looming, and my unintentional reliance on the tool I built, I took the opportunity to spruce things up and host them somewhere I have complete control. The bugs are fixed, a few features were added. I have no idea if anyone but me ever used it, but it lives on!
Enjoy.