body > div {
margin: 100px 100px;
width: 350px;
height: 100px;
background: #000;
color: #7FFF00;
font-family: Arial;
font-weight: 500;
font-size: 3.5em;
text-align: center;
line-height: 100px;
transition: all 0.5s ease;
}
.circle:hover {
border-radius: 100%;
}
<body> <div class="circle">radius</div> </body>