Reader mail: Which 3G data plan should I get for my iPhone?Diane writes: I’m almost all set to buy an iPhone 4 (I’m on AT&T), but I’m stuck on the data plan part. How much do I need, and what would I be using data for, anyway? (And what’s 3G?)

Hi Diane! Great question. Let’s tackle the last question first: why you need a 3G data plan for your iPhone.

Wireless data is essential for mobile activities like sending and receiving email, browsing the web on the iPhone’s Safari web browser, streaming YouTube videos, and shopping online.

Now, your iPhone has a Wi-Fi receiver that will connect to a home, office, or public Wi-Fi router—and when it’s connected via Wi-Fi, your iPhone can send email, surf, and stream videos without cellular data from AT&T.

But when there’s no Wi-Fi around, your iPhone depends on cellular data networks to do anything online. No wireless data, no Internet.

OK, so what’s a 3G network, then? “3G” refers to so-called third-generation cellular data networks, which the big carriers (like AT&T, Verizon, Sprint, and T-Mobile) began rolling out about six or seven years ago.

What’s so special about 3G networks? They’re fast—or at least they are compared to older 2G networks, which are only about as speedy (or pokey, take your pick) as an old-style dial-up network. 3G networks, on the other hand, can download data to your phone several times faster than 2G networks can, while budding 4G networks promise to leave even 3G networks in the dust. (For now, the iPhone 4 only supports 3G data networks.)

So, there’s a little background for you. Now, on to the next question: which 3G data plan should you pick?

On your carrier, AT&T, you have two main choices: 2 GB (short for “gigabyte”) of 3G data for $25 a month, or 200 MB (megabytes) for $15 a month. (Incidentally, Verizon Wireless offers a $30/month unlimited 3G plan for the iPhone, but all indications are that Verizon will soon toss its unlimited plan in favor of metered options similar to those on AT&T.)

Reader mail: Which 3G data plan should I get for my iPhone?

AT&T's online data calculator can help you decide which of the carrier's 3G data plans to pick.

Alright, so how much is 2 GB of data versus 200 MB?

Well, for starters, one gigabyte equals roughly 1,000 megabytes (or 1,024 MB, to be precise), while a MB contains 1,024 kilobytes—details to keep in mind as we consider how much data various online activities consume.

For example: A single, text-only email, for example, averages about 20 kilobytes—meaning you could send and receive more than 5,000 attachment-free email messages each month using the AT&T’s cheaper, 200 MB DataPlus plan.

Not bad, but don’t forget that large email attachments, like photos and PDFs, will add to your monthly total. You’ll also probably want to browse some web pages, too—and at about 200 KB each (depending on the page, of course), you could exceed the monthly limit for the DataPlus package after 600 pages, or 20 webpages a day.

Are you planning on streaming music or video on your iPhone—using, say, mobile apps like Pandora, YouTube, or Netflix? If so, expect to burn through about 500 KB a minute for streaming music, or about 5 MB for every song you download from iTunes. Streaming a two-hour movie on your iPhone over Netflix, meanwhile, could consume close to a gigabyte of data—or about half the monthly allotment for the 2 GB, $25-a-month DataPro plan.

So, how much 3G data might you consume in a month? One handy way to predict your monthly usage is AT&T’s data usage calculator, which lets you adjust a series of sliders depending on how much email, web pages, music, and video you plan on downloading.

As a rule of thumb, though, I tend to tell people who only want to send email and browse the web on their iPhones to go with the 200 MB, $15-a-month DataPlus plan. (Keep in mind that any data you use while connected to a Wi-Fi hotspot won’t count against your monthly 3G allowance.)

If you want to stream music or movies, you’re better off opting for the 2 GB DataPro plan for $25 a month.

OK, but what happens if you sign up for the DataPlus plan and exceed your monthly allowance? Nothing too horrible: you’ll have to pay $15 for another 200 MB bucket of mobile data. Subscribers of the pricier 5GB DataPro plan, meanwhile, will pay an extra $10 for each additional gigabyte over their 2 GB monthly allowance.

Also, remember that you can always change your plan at will, so you could conceivably start with the $15/month DataPlus plan and then move up to the DataPro option if you end up needing more monthly data.

So, Diane … that’s a lot of information, I know, but I hope it helped. Have more questions? Let me know!