calc DEMO

CSS

ul{
	overflow: hidden;
}
li{
	width: -webkit-calc(100%/4);
	width: calc(100%/4);
	float: left;
	line-height: 50px;
	background: #ccc;
	text-align: center;
	list-style-type: none;
}
li:nth-child(even){
	background: #e00;
}
#calc2 li{
	margin: 0 10px;
	width: -webkit-calc(25% - 20px);
	width: calc(25% - 20px);
}

HTML



width:calc(100%/4)指定

width: calc(25% - 20px)指定