Why is div outside of parent div when i use height:100%

Question

I have problems placing a div within another div which has a defined height of lets say 400px.

When i set the inner div to 100% then its overlapping the outer div and goes over the outer one. Why isnt 100% height sizing the inner div to the outer divs height?

body {
  min-width:300px;
}

.header {
  background-color:pink;
  width:100%;
  height:400px;
}

.menu {
  background-color: red;
}

.header-container {
  color:white;
  background-color:gray;
  height:100%;
  max-width:400px;
  margin:auto;
}

.headline {
  padding-right:36px;
  padding-left:36px;
  padding-top:54px;
  padding-bottom:54px;
}

.clearfix {
  clear:both;
}

<div class="header">
  <div class="menu">
  menu
  </div>
  <div class="header-container clearfix">
    <div class="headline">
      <span>das ist mein blog</span>
      <span>this is the underline</span>
    </div>
  </div>
</div>
<div class="blog">
y
</div>
<div class="footer">
x
</div>

https://jsfiddle.net/g9ec4nw8/

Answer 1

Because the the padding on the .header-container is causing an overflow.

Adding box-sizing: border-box; to your .header-container, will fix the box-sizing issue.

But not only that, you haven't taken into account the 18px of height by your .menu.

In full, by changing your .header-container to the following code:

.header-container {
        color:white;
        background-color:gray;
        height:calc(100% - 18px);
        max-width:400px;
        box-sizing: border-box;
        margin:auto;
        top:0;
        bottom:0;
        padding-right:36px;
        padding-left:36px;
        padding-top:54px;
        padding-bottom:54px;
 }

Will fix the issue.

Fiddle Here.