Center a position:fixed element

I would like to make a position: fixed; popup box centered to the screen with a dynamic width and height. I used margin: 5% auto; for this. Without position: fixed; it centers fine horizontally, but not vertically. After adding position: fixed;, it's even not centering horizontally.

Here's the complete set:

.jqbox_innerhtml {
    position: fixed;
    width: 500px;
    height: 200px;
    margin: 5% auto;
    padding: 10px;
    border: 5px solid #ccc;
    background-color: #fff;
}

<div class="jqbox_innerhtml">
    This should be inside a horizontally
    and vertically centered box.
</div>

How do I center this box in screen with CSS?

You basically need to set top and left to 50% to center the left-top corner of the div. You also need to set the margin-top and margin-left to the negative half of the div's height and width to shift the center towards the middle of the div.

Thus, provided a <!DOCTYPE html> (standards mode), this should do:

position: fixed;
width: 500px;
height: 200px;
top: 50%;
left: 50%;
margin-top: -100px; /* Negative half of height. */
margin-left: -250px; /* Negative half of width. */

Or, if you don't care about centering vertically and old browsers such as IE6/7, then you can instead also add left: 0 and right: 0 to the element having a margin-left and margin-right of auto, so that the fixed positioned element having a fixed width knows where its left and right offsets start. In your case thus:

position: fixed;
width: 500px;
height: 200px;
margin: 5% auto; /* Will not center vertically and won't work in IE6/7. */
left: 0;
right: 0;

Again, this works only in IE8+ if you care about IE, and this centers only horizontally not vertically.

I want to make a popup box centered to the screen with dynamic width and height.

Here is a modern approach for horizontally centering an element with a dynamic width - it works in all modern browsers; support can be seen here.

Updated Example

.jqbox_innerhtml {
    position: fixed;
    left: 50%;
    transform: translateX(-50%);
}

For both vertical and horizontal centering you could use the following:

Updated Example

.jqbox_innerhtml {
    position: fixed;
    left: 50%;
    top: 50%;
    transform: translate(-50%, -50%);
}

You may wish to add in more vendor prefixed properties too (see the examples).

Center a position:fixed element

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