Edit in JSFiddle

.parent {
    width: 25em;
    height: 10em;
    margin: 2em auto;
    background: deepskyblue;
}

.child {
    background: tomato;
    width: 100%;
    padding: 1em;
    border: .5em solid;
}

<div class="parent">
    <div class="child">I've both a padding and a border.</div>
</div>