JSFiddle - React, Tailwind, and code Playground

HTML

úvěr: <input type="number" id="souc_hod" value="2000000" oninput="spocti()"><br>
sazba: <input type="number" id="sazba" value="3" oninput="spocti()"><br>
počet splátek: <input type="number" id="pper" value="360" oninput="spocti()"><br>
měsíční splátka = <span id="vysledek">8433</span> Kč

JavaScript

function platba(sazba, pper, souc_hod) {
 return sazba * souc_hod * Math.pow((1 + sazba), pper) / (1 - Math.pow((1 + sazba), pper));
}

function spocti(){
var souc_hod = document.getElementById('souc_hod').value;
var sazba = document.getElementById('sazba').value/100/12;
var pper = document.getElementById('pper').value;
document.getElementById('vysledek').textContent = Math.ceil(platba(sazba, pper, souc_hod)*-1);
}