JumpStart and many themes have this built in, but just in case we ever need it.
CSS:

.video {
	 position: relative;
	 padding-bottom: 56.25%;
	 padding-top: 20px;
	 height: 0;
	 overflow: hidden;
}
.video iframe,  
.video object,  
.video embed {
	 position: absolute;
	 top: 0;
	 left: 0;
	 width: 100%;
	 height: 100%;
}

HTML:

<div class="video">
<iframe width="560" height="315" src="VIDEO URL HERE" frameborder="0" allowfullscreen></iframe>
</div>

for more info click