Setting a line-height
the same value as the height of the div will show one line of text vertically centered. In this example the height and line-height are 500px.
Example
.circle {
width: 500px;
height: 500px;
line-height: 500px;
border-radius: 50%;
font-size: 50px;
color: #fff;
text-align: center;
background: #000
}
<div class="circle">Hello I am A Circle</div>