Use calc
!
https://jsbin.com/wehixalome/edit?html,css,output
HTML:
<div class="left">
100 px wide!
</div><!-- Notice there isn't a space between the divs! *see edit for alternative* --><div class="right">
Fills width!
</div>
CSS:
.left {
display: inline-block;
width: 100px;
background: red;
color: white;
}
.right {
display: inline-block;
width: calc(100% - 100px);
background: blue;
color: white;
}
Update: As an alternative to not having a space between the divs you can set font-size: 0
on the outer element.