I use simple algebra and occasionally a bit of geometry estimating and managing construction projects, but I’ve never found it necessary to find an exact solution for an area under a curve or the rate of change of a function at a specific point.

I don’t know how to solve an integral, but I understand that an electricity meter or flowmeter (gas and air) is performing continuous integration to keep track of total power usage or fluid flow. That’s good enough for me, and if I had to I could approximate a solution with a piece of paper, a pair of scissors, and a reasonably accurate scale; or trapezoids and graph paper.

Re: your edit, calculus would definitely come in handy if you had an interest in RF. The good thing is, you could still learn it :)