vertically and horizontally center a fixed div

.parent {
  position: relative;
}
.child {
    position: absolute;
    left: 50%;
    top: 50%;
    transform: translate(-50%, -50%);
}

.container{	
	margin: 0;
    position: absolute;
    top: 50%;
    left: 50%;
    -ms-transform: translate(-50%, -50%);
    transform: translate(-50%, -50%);
  	width: /* Define the width here; */
    height: /* Define the height here; */
}

/*You have to define the width and height! */

// you dont need to know the divs dimensions and it does not require any container divs
position: fixed; 
top: 50%;
left: 50%;
transform: translate(-50%, -50%);

left: 50%;
transform: translateX(-50%);

left: 50%;
margin-left: -400px; /* Half of the width */