this should do the trick
Code:
.calc {
width: 100px;
height: 100px;
border: 1px solid #f00;
padding: 10px;
/* Firefox */
width: -moz-calc(75% - 100px);
/* WebKit */
width: -webkit-calc(75% - 100px);
/* Opera */
width: -o-calc(75% - 100px);
/* Standard */
width: calc(75% - 100px);
}