<html lang="en"><head> <meta charset="utf-8"> <title>calc function</title> <style> .wrapper { width: 400px; height:100px; border:1px solid gray; } .box { width: calc(20% + 100px); border:5px solid purple; } </style></head><body> <div class="wrapper"> <div class="box">My width is calculated.</div> </div></body></html>