Rechner

<!DOCTYPE html>
<html>
<head>
    <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css">
</head>
<body>
    <form class="col-lg-6 offset-lg-3">
        <h1>Rechner</h1>
        <div class="form-group">
            <label for="income">Netto-Einkommen</label>
            <input id="income" class="form-control" type="text" value="30000" name="einkommen" autocomplete="off">
        </div>
        <div class="form-group">
            <label for="expenses">Ausgaben pro Jahr</label>
            <input id="expenses" class="form-control" type="text" value="12000" name="ausgaben" autocomplete="off">
        </div>
        <div class="form-group">
            <label for="savings">Sparbetrag pro Jahr</label>
            <input id="savings" class="form-control" type="text" value="18000" name="sparrate" autocomplete="off">
        </div>
        <div class="form-group">
            <label for="networth">Aktuelles Vermögen</label>
            <input id="networth" class="form-control" type="text" value="0" name="vermoegen" autocomplete="off">
        </div>
        <div class="form-group">
            <label for="roi">Jährliche Rendite</label>
            <input id="roi" class="form-control" type="text" value="5" name="rendite" autocomplete="off">
        </div>
        <div class="form-group">
            <label for="withdrawal-rate">Entnahmerate</label>
            <input id="withdrawal-rate" class="form-control" type="text" value="4" name="withdrawal" autocomplete="off">
        </div>
        <input type="button" value="Berechnen">

        <br><br>
        <p id="output"></p>
    </form>

    <script type="text/javascript">
        const button = document.querySelector('input[type="button"]');
        const output = document.querySelector('p#output');

        button.addEventListener('click', calculate);

        function calculate() {
            var income = document.getElementById('income').value;
            var expenses = document.getElementById('expenses').value;
            var savings = document.getElementById('savings').value;
            var networth = document.getElementById('networth').value;
            var roi = document.getElementById('roi').value / 100;
            var withdrawalrate = document.getElementById('withdrawal-rate').value / 100;

            var savingrate = (income - expenses) / income;

            if(withdrawalrate == 0 || savingrate == 0) {
                output.textContent = 'So wird das nichts...';
            }
            else {
                var years =  Math.log((((roi * (1 - savingrate) * income) / withdrawalrate) + (savingrate * income)) / ((roi * networth) + (savingrate * income))) / Math.log(1 + roi);

                var years_string = Math.round(years *10)/10;
                var savingrate_string = Math.round(savingrate*100)+'%'

                output.textContent = 'Du kannst in Rente gehen in '+years_string+' Jahren bei einer Sparquote von '+savingrate_string+'.';
            }
        }
    </script>
</body>
</html>