How can I make button auto width to its parent div

I can’t make a button auto its width to the parent div it is a child of. How do I make that?

.

Here is the code (html structure) if you need it:

<div class="drp">
  <button class="drp-btn" id="i-drp-btn">m/s</button>
  <div id="i-speed-drp" class="drp-content">
    <button class="drp-btn-elem" value="m/s">m/s</button>
    <button class="drp-btn-elem" value="km/h">km/h</button>
  </div>
</div>

>Solution :

Did you try width: 100% on your button style?
It should take the size of the parent, except if you have margin defined on your parent or padding defined on your button.