I am trying to add a volume calculator and I can't figure out how this code is broken... I think it's missing escaped special characters or a tag...
<h1>Cubic meter (m³) Calculator</h1>
<input id="length" placeholder="5"><br>
<input id="width" placeholder="5"> <br>
<input id="thickness" placeholder="5"><br>
<p>Cubic Meters (m³)</p>
Only HTML per se won't be able to do cubic volume calculations, nor any type of calculation at all.
It seems to me you are not familiarized with coding, in this case you'd want to hire a developer to be able to implement this type of feature into your Shopify store. Alternatively you can look for an app that already achieves that. This is relatively complex, so it's unlikely (yet definitely not impossible) that someone around here will devote their time to get it coded in for you. Wishing you all the best.