volume calculator options?

So i have an aquarium volume calculator on my site that is JavaScript. A few members on this site helped me make it. I want to have a dropdown menu so people can select the units like inches and feet. I am not very good with javascript. Another member on here who helped me get started with the calculator added a reference to a share script on fishlore, another aquarium forum that is not mine and why is that there? Here is the code i am using now:

<script type="text/javascript" src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.1/jquery.min.js"></script>
<script type="text/javascript" src="http://www.fishlore.com/scripts/js/share.js"></script>
<script type="text/javascript">
var result = 0;
var roundLength = 2;
  function computeGallons()
{
    result = Math.round((document.frmMain.TankLength.value * document.frmMain.TankWidth.value * document.frmMain.TankHeight.value / 231)*100)/100;
    document.frmMain.USGallons.value = Math.round(result*100)/100;
}
</script>
<form name="frmMain"><span class="b">Aquarium Calculator: Calculate Rectangle Fish Tank Volume in Gallons Based on Tank Size</span><p>
 
        Fish Tank Length in Inches:<br />
        <input type="text" name="TankLength" onkeyup="computeGallons()"><br />
        <br />
        Fish Tank Width in Inches:<br />
        <input type="text" name="TankWidth" onkeyup="computeGallons()"><br />
        <br />
        Fish Tank Height in Inches:<br />
        <input type="text" name="TankHeight" onkeyup="computeGallons()"><br />
        <br />
        Aquarium Size is<br />
        <input type="text" name="USGallons" style="border: none; background: #D8EFFF; padding: 12px; width: 10em; font-size: 18px;" readonly> US Gallons<br />
        <br />
     
</form>

Code (JavaScript):