JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" name="M_Fritt_Klor"/><br>
<input type="text" name="M_Total_Klor"/><br>
<input type="text" name="M_Bundet_Klor" disabled />

JavaScript

$(function() {
  var inputs = $('input[name="M_Fritt_Klor"],input[name="M_Total_Klor"]');
      output = $('input[name="M_Bundet_Klor"]');
    function parse(value) {
        return parseFloat(value.replace(',','.'));
    }
  inputs.keyup(function() {
      var value = parse(inputs[0].value)-parse(inputs[1].value);
      if (!isNaN(value)) {
          output.val(value.toString().replace('.', ','));
      }
  });
});