I am working on a calculation form, but I can't get it to work right. The formula that has to be calculated is prefa = plaho * (plakg + 2*grekg).
When I try to calculate twegrekg it works, but there is something wrong when I try to calculate plageto.
function calc() {
var plaho = document.getElementById('plaho').value;
var plakg = document.getElementById('plakg').value;
var grekg = document.getElementById('grekg').value;
var twegrekg = 2 * grekg;
var plageto = twegrekg + plakg;
var prefa = plageto * plaho;
document.getElementById('result').innerHTML = prefa;
return false
}<form>
<p>Plaathoogte (in mm)<input type="number" id="plaho" /></p>
<p>Plaatgewicht (in kg)<input type="number" id="plakg" /></p>
<p>Greepgewicht (in kg)<input type="number" onChange="calc()" id="grekg" /></p>
<input type="button" onClick="calc()" value="Bereken" />
</form>
<div id="result"></div>