Positioning a div inside of an overlay - html

Question

Trying to create a contact form in an overlay. I have the overlay div working, but it only works if I set it to "position: absolute". The div inside of the overlay, will not position properly, regardless of what I try.
Need the "form-wrap to be centered vertically & horizontally.
<div id="overlay">
<div id="form-wrap">
<img id="close-btn" src="images/framework/close.png">
<form id="form-box">
<input name="first-name" id="first-name" type="text" maxlength="32">
</form>
</div>
</div>
#overlay{
top: 0;
left: 0;
height:100%;
width: 100%;
position: absolute;
z-index: 899;
display: none;
background-color: rgba(0, 0, 0, 0.41);
}
#form-wrap{
width: 400px;
height: 600px;
position: relative;
z-index: 900;
margin: 0;
background-color: white;
}

Answer 1

try this:
#overlay{
top: 0;
left: 0;
right: 0;
bottom: 0;
position: fixed;
z-index: 899;
display: none;
background-color: rgba(0, 0, 0, 0.41);
}
#form-wrap{
width: 400px;
height: 600px;
position: relative;
top: 50%;
left: 50%;
margin: -300px 0 0 -200px;
background-color: white;
}
Example

Answer 2

This will only work if the height of your overlay is greater than 600px!
#overlay{
top: 0;
left: 0;
height: 100%;
width: 100%;
position: absolute;
z-index: 899;
display: none;
background-color: rgba(0, 0, 0, 0.41);
}
#form-wrap{
width: 400px;
height: 600px;
top: 50%;
position: relative;
z-index: 900;
margin: -300px auto 0 auto;
background-color: white;
}
Example