Any way to do "real-time" unit conversion in web application

I want to build a website that allows the user to convert Hz to bpm (note however that my question concerns all sort of unit conversion). I am using PHP and am hosting my website on Apache.

So, I was wondering if it was possible to "bind" an input field to an HTML element, like we do in WPF developement where you can bind an input area to a WPF element, and apply some sort of data conversion so if the user types 12 bpm, the bound element will automaticly and immediately display 0.2 Hz. If the user then adds a "0" so 120 bpm will be automaticly and immediately converted to 2 Hz.

The effect I am trying to describe can also be noted on this very forum : as I type my question, I can see a "real-time" final version of my text.

How is this achieved? Is there any way to do it with PHP? I know of AJAX but I would really prefer to avoid using Javascript to hold my math functions. Correct me if I am wrong but I think this could be accomplished with Node.js? Should I consider migrating to Node?

With just the DOM and JavaScript, you can use the input event on a text field to receive an immediate callback when its value changes as the result of user action:

document.querySelector("selector-for-the-field").addEventListener("input", function() {
    // Code here to do and display conversion
}, false);

Example (centigrade/Celsuis to Fahrenheit):

var f = document.getElementById("f");
document.querySelector("#c").addEventListener("input", function() {
  var value = +this.value;
  if (!isNaN(value)) {
    value = (value * 9) / 5 + 32;
    f.innerHTML = value;
  }
}, false);

<div>
  <label>
  Enter centigrade:
  <input type="text" id="c">
</label>
</div>
<div>
  Fahrenheit: <span id="f"></span>
</div>

Stepping back, yes, there are dozens of libraries and frameworks that provide MVC/MVVM/MVM/whatever-the-acronym-is-this-week in the browser. A short list of current popular ones: React, Angular, Vue, Knockout, ... Note that these are not magic, they're just code written in JavaScript (or something like TypeScript or Dart or CoffeeScript that compiles to JavaScript) that use the DOM the covers.