<div class="box content-box">content-box / width: 500px / padding: 20px / border: 5px</div>
<div class="box border-box">border-box / width: 500px / padding: 20px / border: 5px</div>

.box {
  width: 500px;
  height: 100px;
  padding: 20px;
  margin: 20px;
}

.content-box {
  box-sizing: content-box;
  background-color: lightblue;
  border: 5px solid blue;
}

.border-box {
  box-sizing: border-box;
  background-color: lightpink;
  border: 5px solid red;
}