How to force child div to be 100% of parent div's height without specifying parent's height?

Question

I have a site with the following structure:

  

  <         


        
      

Answer 1

I had the same issue, it worked based on Hakam Fostok's answer, I've created a small example, in some cases it might work without having to add display: flex; and flex-direction: column; on the parent container

.row {
    margin-top: 20px;
}

.col {
    box-sizing: border-box;
    border: solid 1px #6c757d;
    padding: 10px;
}

.card {
    background-color: #a0a0a0;
    height: 100%;
}

JSFiddle