This is a related problem that people might come to this page when searching: When I want to centre a div for a (100px square) "waiting.." animated gif I use :
.centreDiv {
position: absolute;
top: calc(50vh - 50px);
top: -moz-calc(50vh - 50px);
top: -webkit-calc(50vh - 50px);
left: calc(50vw - 50px);
left: -moz-calc(50vw - 50px);
left: -webkit-calc(50vw - 50px);
z-index: 1000; /*whatever is required*/
}