Edit in JSFiddle

<input 
  type="text" 
  placeholder="without box-sizing">

<input
  type="text" 
  placeholder="with box-sizing">
body {
  width: 200px;
  margin: 0 auto;
  border-left: 1px #ccf solid;
  border-right: 1px #ccf solid;
}

input {
  width: 100%;
  padding: 10px;
  margin: 30px 0;
  font-size: 1em;
}

input + input {
  box-sizing: border-box;
}