Keeping an eye on your data usage is an important element in controlling your wireless bill. Most data plans offered with devices like the iPhone have a hard cap -- if you go over the cap, you'll be charged extra for the data you used. While every iPhone user's data usage is different, Safari and YouTube both demand access to the Web either through your data plan or via Wi-Fi.

Safari and Data

Safari is the iPhone's built-in Web browser. Any website you visit through Safari, whether you're reading the news or looking at adorable kitten pictures, uses data. The exact amount of data used depends on the site; media-rich sites with videos and images will use large amounts of data while mobile-friendly or text-only sites will use less. Generally speaking, regular Web browsing is not as intensive on your data connection as watching and downloading videos.

YouTube and Data

In order to watch any video on YouTube, the video must first be downloaded via the Web. YouTube downloads videos as you watch them. Data used for watching a YouTube video depends on the video quality and video length. Higher qualities mean more data, as do longer videos. YouTube videos on mobile average out to about 2-3MB per minute. If you watch lots of YouTube on your iPhone, you're probably using much more data than you would browsing the Web in Safari.

Wi-Fi Connections

You only use data when your iPhone is connected to your cellular network. If you're browsing the Web or watching videos while the phone is connected to your Wi-Fi network, that data is being relayed over your home network and will not count against the data usage limits in your cellular contract. Avoid high data usage by enabling Wi-Fi on your iPhone whenever possible -- you can even use Wi-Fi to text with Apple's iMessage service.

Monitor Your Usage

Monitoring your data usage will help prevent you from accidentally going over your limits. Log in to your user account on your cellular service provider's website to track usage through their utilities, or use the built-in iPhone data tracker to get a rough idea of where you stand. Tap the “Settings” icon, select “Cellular” and then scroll down to see how much data you've used. You can also use this menu to disable cellular access for specific apps.