You can use calc, modern browsers support it and IE9+ as well.
div {
margin: 10px;
width: calc(100% - 20px);
height: 10px;
background: teal;
}<div></div>Browser support
You can use calc, modern browsers support it and IE9+ as well.
div {
margin: 10px;
width: calc(100% - 20px);
height: 10px;
background: teal;
}<div></div>Browser support