JSFiddle - React, Tailwind, and code Playground

HTML

<input name="pedagio" type="text" class="form-control" id="pedagio" value="" required>

JavaScript

$(document).ready(function () {
    $("#pedagio").change(function () {
        var peso = parseFloat(this.value);
        var pedagio = 6.42;
        var extra = Math.ceil(peso / 100) * pedagio;
        this.value = peso + extra;
    });
});