CSS layout with fixed top and bottom, variable height middle

Solution 1:

Position the middle div using absolute positioning without specifying height. It does not get much simpler than this:

#header {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    height: 100px;
    background-color: #abcdef;
}
#footer {
    position: fixed;
    bottom: 0;
    left: 0;
    right: 0;
    height: 100px;
    background-color: #abcdef;
}
#content {
    position: fixed;
    top: 100px;
    bottom: 100px;
    left: 0;
    right: 0;
    background-color: #F63;
    overflow: auto;
}

<div id="header"></div>
<div id="content"></div>
<div id="footer"></div>

Use "Full page" option to view the snippet properly.

Solution 2:

If you don't know the header or footer sizes and you can use CSS3 then i would suggest to use flexbox layouting.

Example below (or check fiddle)

HTML:

<div class="container">
<div class="header">header</div>
<div class="content">content</div>
<div class="footer">bottom</div>
</div>

CSS:

.container {
  display: flex;
  flex-direction: column;
  width: 100%;
  height: 400px;
}

.header {
  flex-grow: 0;
  background-color: red;
}
.content {
  flex-grow: 1;
  background-color: green;
}
.footer {
  flex-grow: 0;
  background-color: blue;
}