Edit in JSFiddle

.calculated-width {
    width: -moz-calc(100% - 100px);
    width: -webkit-calc(100% - 100px);
    width: calc(100% - 100px);
}
.well {
    background: #ffa;
}

<div class="well calculated-width">
    this block has <strong>width = 100% - 100px</strong>
</div>