首先，我不熟悉这个。我想做一个简单的表格来计算右矩形棱镜的体积。如果用户选择“自定义”选项，则用户可以自定义长度（自定义盘江）、宽度（自定义列巴）和高度（自定义丁吉）。但也有其他不需要定制的选项（10x10x5）等。
我找到了通过自定义选项计算结果的解决方案。但是我如何计算其他选项（不计算）。

function pilihUkuran() {
  var custom = document.getElementById("ukuran");
  if (custom.value == "30x30x5") {
    document.getElementById("customukuran1").style.visibility = "hidden";
  } else if (custom.value == "10x10x5") {
    document.getElementById("customukuran1").style.visibility = "hidden";
  } else if (custom.value == "20x20x5") {
    document.getElementById("customukuran1").style.visibility = "hidden";
  } else
    document.getElementById("customukuran1").style.visibility = "visible";
}

function mult(pj, lb, tg) {
  var isipj = pj;
  var isilb = lb;
  var isitg = tg;
  var hasil = isipj * isilb * isitg;
  document.getElementById('rupiah').value = hasil;
}

<div>
  <label class="col-sm-2 mb-3">&nbsp;&nbsp;Ukuran</label>
  <select name="ukuran" id="ukuran" class="form-select-auto" onchange="pilihUkuran()" placeholder="Input ukurannya" method="post" required>
    <option name="ukuran_1" id="ukuran_1" value="custom" method="post">Custom</option>
    <option name="ukuran_2" id="ukuran_2" value="10x10x5" selected>10x10x5 cm</option>
    <option name="ukuran_3" id="ukuran_3" value="20x20x5">20x20x5 cm</option>
    <option name="ukuran_4" id="ukuran_4" value="30x30x5">30x30x5 cm</option>
  </select>
</div>

<div id="customukuran1" style="visibility: hidden;">
  <select name="customukuran" id="customukuran" class="form-control" style="display:none;" method="get">
    <!-- MENDAPATKAN VARIABEL P x L x T -->
    <input class="customp" id="custom_panjang" onkeyup="mult(this.value,document.getElementById('custom_lebar').value,document.getElementById('custom_tinggi').value);" name="custom_panjang" type="number" placeholder="Panjang (cm)">

    <input class="customl" id="custom_lebar" onkeyup="mult(document.getElementById('custom_panjang').value,this.value,document.getElementById('custom_tinggi').value);" name="custom_lebar" type="number" placeholder="Lebar (cm)">

    <input class="custom3" id="custom_tinggi" onkeyup="mult(document.getElementById('custom_panjang').value,document.getElementById('custom_lebar').value,this.value);" name="custom_tinggi" type="number" placeholder="Tinggi (cm)">

  </select><br>
</div>

<input class="mr-5 rupiah" name="rupiah" id="rupiah" disabled style="color: black; font-weight: bolder;">

或者你可以在这里看到演示https://jsfiddle.net/aurj74nq/
请帮帮我，谢谢。