Flex vertical alignment

HTML

<div class="box-holder">
  <div class="flex"></div>
</div>

CSS

.box-holder {
    display: flex;
    height: 100vh;
    width: 100%;
    align-items: center;
    justify-content: center;
}
.flex {
    border: 1px solid gray;
    height: 10%;
    width: 10%;
}