fit-content

<a class="btn1" href="">ボタン</a>

.btn2 {
  padding: 10px 40px;
  background-color: #ccc;
  display: block;
}

<a class="btn2" href="">ボタン</a>

.btn2 {
  width: fit-content;
  padding: 10px 40px;
  margin: 0 auto;
  background-color: #ccc;
  display: block;
}