Parent Height doesn't follow their float children [duplicate]

Answer 1

Use overflow: hidden and clear a float

#mainContainer{
    width: 1000px;
    height:auto;
    margin-left:auto;
    margin-right:auto;
    background-color: #ff6600;
    padding-bottom: 20px;

    overflow: hidden;
    clear: both;
}

Answer 2

You need to clear your floating elements, use overflow: hidden; for #mainContainer

Demo

Alternate : (You can add clear: both; to clear floats)

Demo

Or you can also self clear floating elements (Only if you wish to support IE9=+

.clear:after {
  content: "";
  clear: both;
  display: table;
}

Why this happens?

Answer 3

Add overflow:hidden; to the container:

#mainContainer{
    width: 1000px;
    /*height: 1000px;*/
    height:auto;
    margin-left:auto;
    margin-right:auto;
    background-color: #ff6600;
    padding-bottom: 20px;

    overflow: hidden; /* <--- here */
}

Because its content is floated, the container div collapses. Using a 'clearfix' class or, as I mentioned, adding overflow:hidden will cause the container to contain the floated elements.

UPDATE Explanation of why this works can be found here: https://stackoverflow.com/a/9193270/1588648

... and here:

In order for them (browsers) to calculate what overflowed the bounds of the block (and thus should be hidden), they needed to know the size of the block. Because these blocks do no have an explicit height set, the browsers used the calculated height of the content instead.

http://www.stubbornella.org/content/2009/07/23/overflow-a-secret-benefit/