Source

Sizing

Easily make an element as wide or as tall (relative to its parent) with our width and height utilities.

Width and height utilities are generated from the $sizes Sass map in _variables.scss. Includes support for 25%, 50%, 75%, 100%, and auto by default. Modify those values as you need to generate different utilities here.

Width 25%
Width 50%
Width 75%
Width 100%
Width auto 
<div class="w-25 p-3" style="background-color: #eee;">Width 25%</div>
<div class="w-50 p-3" style="background-color: #eee;">Width 50%</div>
<div class="w-75 p-3" style="background-color: #eee;">Width 75%</div>
<div class="w-100 p-3" style="background-color: #eee;">Width 100%</div>
<div class="w-auto p-3" style="background-color: #eee;">Width auto</div>
Height 25%
Height 50%
Height 75%
Height 100%
Height auto 
<div style="height: 100px; background-color: rgba(255,0,0,0.1);">
  <div class="h-25 d-inline-block" style="width: 120px; background-color: rgba(0,0,255,.1)">Height 25%</div>
  <div class="h-50 d-inline-block" style="width: 120px; background-color: rgba(0,0,255,.1)">Height 50%</div>
  <div class="h-75 d-inline-block" style="width: 120px; background-color: rgba(0,0,255,.1)">Height 75%</div>
  <div class="h-100 d-inline-block" style="width: 120px; background-color: rgba(0,0,255,.1)">Height 100%</div>
  <div class="h-auto d-inline-block" style="width: 120px; background-color: rgba(0,0,255,.1)">Height auto</div>
</div>

You can also use max-width: 100%; and max-height: 100%; utilities as needed.

Max-width = 100% [1000%x100] 
<img class="mw-100" src=".../1000px100?text=Max-width%20%3D%20100%25" alt="Max-width 100%">
Max-height 100% 
<div style="height: 100px; background-color: rgba(255,0,0,0.1);">
  <div class="mh-100" style="width: 100px; height: 200px; background-color: rgba(0,0,255,0.1);">Max-height 100%</div>
</div>