How to center a button within a div?

Updated Answer

Updating because I noticed it's an active answer, however Flexbox would be the correct approach now.

Live Demo

Vertical and horizontal alignment.

#wrapper {
  display: flex;
  align-items: center;
  justify-content: center;
}

Just horizontal (as long as the main flex axis is horizontal which is default)

#wrapper {
  display: flex;
  justify-content: center;
}

Original Answer using a fixed width and no flexbox

If the original poster wants vertical and center alignment its quite easy for fixed width and height of the button, try the following

Live Demo

CSS

button{
    height:20px; 
    width:100px; 
    margin: -20px -50px; 
    position:relative;
    top:50%; 
    left:50%;
}

for just horizontal alignment use either

button{
    margin: 0 auto;
}

div{
    text-align:center;
}

You could just make:

<div style="text-align: center; border: 1px solid">
  <input type="button" value="button">
</div>

Or you could do it like this instead:

<div style="border: 1px solid">
  <input type="button" value="button" style="display: block; margin: 0 auto;">
</div>

The first one will center align everything inside the div. The other one will center align just the button.

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