CSS center a div thats over top of an overlay - html

Question

I created an overlay with div over top of it, right now I am trying to center the div over top of the overlay, I tried messing around the left and right but nothing worked.
.request-estimate {
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
z-index: 99999;
background-color: rgba(0,0,0,0.8);
display: none;
}
.request-estimate-box {
display: none;
height: 400px;
width: 40%;
margin: 0 auto;
background-color: #FFF;
z-index: 99999;
position: fixed;
top: 15%;
padding: 20px;
border-radius: 5px;
}
And here is the CSS
<div class="request-estimate"></div>
<div class="request-estimate-box">
<h1>Request Free Estimate</h1>
<form action="" method="post">
<p>
<label for="name">Name</label>
<input type="text" name="name" id="name" class="form-control" />
</p>
<p>
<label for="email">Email</label>
<input type="email" name="email" id="email" class="form-control" />
</p>
<p>
<label for="phone">Phone</label>
<input type="phone" name="phone" id="phone" class="form-control" />
</p>
<p>
<input type="submit" name="submit" id="submit" value="Submit" class="btn btn-default" />
</p>
</form>
</div>

Answer 1

.request-estimate {
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
z-index: 99999;
background-color: rgba(0,0,0,0.8);
/*display: none;*/
}
.request-estimate-box {
/*display: none;*/
height: 400px;
width: 40%;
margin-left:-20%; /*add this = width/2*/
background-color: #FFF;
z-index: 99999;
position: fixed;
top: 15%;
left:50%; /*add this*/
padding: 20px;
border-radius: 5px;
}
<div class="request-estimate"></div>
<div class="request-estimate-box">
<h1>Request Free Estimate</h1>
<form action="" method="post">
<p>
<label for="name">Name</label>
<input type="text" name="name" id="name" class="form-control" />
</p>
<p>
<label for="email">Email</label>
<input type="email" name="email" id="email" class="form-control" />
</p>
<p>
<label for="phone">Phone</label>
<input type="phone" name="phone" id="phone" class="form-control" />
</p>
<p>
<input type="submit" name="submit" id="submit" value="Submit" class="btn btn-default" />
</p>
</form>
</div>

Answer 2

Solution: https://jsfiddle.net/gqqqeLfL/
request-estimate-box was not being contained by request-estimate.
Originally:
<div class="request-estimate"></div>
<div class="request-estimate-box"></div>
Changed to:
<div class="request-estimate">
<div class="request-estimate-box"></div>
</div>
Furthermore, removed position:fixed since that glues the element to an exact position thus nullifing your margin-auto. Now the position default is position:relative and the margin:auto is properly executed.

Answer 3

I see two alternatives :
.request-estimate-box {
height: 400px;
width: 40%;
background-color: #FFF;
z-index: 99999;
position: fixed;
top: 15%;
left: 0;
right: 0;
margin: auto;
padding: 20px;
border-radius: 5px;
}
http://codepen.io/anon/pen/dPLxPJ?editors=110
And :
.request-estimate-box {
height: 400px;
width: 40%;
background-color: #FFF;
z-index: 99999;
position: fixed;
top: 15%;
left: 50%;
-webkit-transform: translateX(-50%);
transform: translateX(-50%);
padding: 20px;
border-radius: 5px;
}
http://codepen.io/anon/pen/ByEXyM?editors=110
And a margin half it's width from the center of course, like Tambo is suggesting.