Edit in JSFiddle

Result
CSS
HTML

.ex1 {
    width: 320px;
    height: 30px;
    padding: 10px;
    border: 5px solid pink;
    margin: 20px;
}
.ex2 {
    width: 320px;
    height: 30px;
    padding: 10px;
    border: 5px solid pink;
    margin: 20px;
    box-sizing: border-box;
}

<!DOCTYPE html>
<html lang="ko">
  <head></head>
  <body>
    <div class="ex1">content-box 테두리, 패딩 제외</div>
    <div class="ex2">border-box 테두리, 패딩 포함</div>
  </body>
</html>