CSS

circle button html

.btn {
  height: 300px;
  width: 300px;
  border-radius: 50%;
  border: 1px solid red;
}

circle button css

.btn{
    width: 50px;
    height:50px;
    border-radius: 50%;
    border: none;
    color: white;
    background: #dc3545; 
    padding: 0px;
    text-align:center;
}