Leaving 3G and mobile data enabled on your iPhone means you might go over your monthly data limit. Here we explain how to turn off 3G and data, and disable data roaming as well.
Since it can be a bit confusing, let's start by explaining the difference between mobile data, 3G and data roaming.
Mobile data is best thought of as mobile internet. If you switch off mobile data, you won't be able to browse the web or use any app that relies on a live internet connection. Some apps, typically email, contacts and calendars will have some functions when offline, but may not be fully usable until you have an internet connection again.
Switching off mobile data doesn't mean you can't use the internet on your iPhone at all. As long as you have a Wi-Fi connection, you'll be able to browse the web and email people as normal.
3G refers to the speed of your mobile internet connection. If you have the option (those on 3 won't, as it's a purely 3G network), you can disable 3G and revert to '2G' which is the slower GPRS or EDGE standards.
Turning off the 3G data connection will help prolong battery life and still allow you to make and receive calls and access data using EDGE or GPRS where available.
Data roaming means using mobile data in another country. Operators charge considerably more for using the internet abroad, so it makes sense for most people to leave their iPhone's data roaming option set to 'off' and find a cheaper Wi-Fi hotspot to check emails and browse the web.
How to turn off 3G and mobile data on an iPhone
In this guide, we assume your phone is running iOS 6, but the settings are very similar for older versions of iOS. Depending on your mobile operator, the names of some options may be different to those you see in the screenshots below (a phone using O2 in the UK on a pay-as-you-go tariff).
From the home screen tap Settings (if you can't see the Settings app, swipe right to get to the search screen - Spotlight - and search for 'settings').
Select General and then choose NMobile Data. On some iPhones, this might be called Cellular instead. On older versions of iOS, look for Network.
Now you can use the the slider next to Enable 3G to turn it off. However, if you are using a mobile operator that supports only 3G (such as Three), you won't see this option at all: you can't disable 3G.
You can also disable Mobile Data from here, which means you must use Wi-Fi for internet access. Similarly, you can choose whether to allow data roaming.
Scroll down on the same screen and you'll see a set of options for restricting what mobile data can be used for. Some apps will warn you independently that you might not want to perform a task because it could use a lot of mobile data. Apple's latest Podcasts app will ask before using mobile data to stream or download anything, for example.