So what is a gigabyte for mobile users, anyway?

Verizon’s (s vz) move this week to follow rival AT&T (s t) by eliminating unlimited data plans may not affect most smartphone users today, but could impact them in the future as handsets are used for more purposes. Such tiered data plans, where customers buy a set amount of monthly broadband, are easy to understand at the point of purchase, what isn’t clear is how much data is needed for different activities. Deciding which data plan will be enough for a month of smartphone use, without choosing a higher priced plan that provides more bandwidth than is needed represents a growing challenge.

To help its customers adjust from unlimited plans, Verizon has created an online calculator with a number of data-intensive activities, which complements but differs from AT&T’s own data usage calculator.¬†And that’s the crux of the problem here, because different smartphone uses eat up different amounts of bandwidth in a given time. Think of the system like electricity, although in this case, you have to measure by the megabyte per hour instead of a kilowatt per hour. You can light a room with 100 watt incandescent bulb and use 100 watts in a given hour or you can illuminate the same room with a more modern LED light bulb using (and paying for) only 13 watts of power in a hour. The same concept applies to emails, web browsing, VoIP calls, and streaming video: All use the same network, but require different amounts of broadband.

Verizon’s calculator does a fine job at providing a monthly estimate of broadband data needs based on the wide range of mobile activities, but here are some guidelines on just how much data different activities use on a smartphone. Bear in mind that these are general guidelines as the broadband needs of some similar activities will vary, which makes this all the more complicated. Streaming the same video in a high quality or high-resolution setting will use more bandwidth than streaming the same video streamed at a lower setting, for example.

Mobile broadband usage by activity
Activity Data use in Megabytes (MB)
Streaming low quality music (64 kbps / 1 hour) 28.8
Streaming high quality music (192 kbps / 1 hour) 86.4
Low quality video (1 hour) 200
High definition video (1 hour) 400
Upload / download a 5 megapixel photo (JPEG) 1.5
Upload / download a 1080p video (1 hour) 2,000
Upload / download a 720p video (1 hour) 1,000
Download an average 400 page ebook 0.77
Video call (1 hour) 75
Typical email, text only 0.01
Install Angry Birds on Android 19
Install Need for Speed Shift on iPhone 179
Download a 42 minute album from iTunes 85
Note: 1,000 MB = 1 GB

Again, these are guidelines to get you started in understanding how much mobile broadband smartphone activity requires. Because some activities use small bits of data, such as social networking status updates, tweets and text-only instant messaging, I’ve bypassed them in this table. But data-intensive activities, like those in the chart, can add up. A 2 GB monthly plan, could be used up in as little as a few hours if you stream high-quality video to a smartphone, for example.

There are a number of varying factors when considering mobile broadband needs, but one mantra should hold true: The higher the quality or the more immersive the experience of any media or file, the more bandwidth it will require. That’s just like the electricity example. If you want brighter lights in a room, you’re going to need more electricity.

It’s also worth noting that the more mobile broadband you purchase under either the AT&T or Verizon plan options, the cheaper the service is on a per-gigabyte basis. At the low end, a 200 MB plan, or one-fifth of a gigabyte is $15 or $30, depending on the carrier. That works out to a per-gigabyte cost of $125 to $150. But a 2 GB plan from AT&T is $25, making the price of a gigabyte a much more palpable $12.50. And a whopping 10 GB from Verizon is $80, bringing the per-gigabyte unit price down to $8. Overages for both carriers in most cases are $10 per gigabyte, which happens to be very close to what it costs a carrier to deliver a gigabyte of mobile broadband, according to Chetan Sharma, a wireless analyst.

Note: the table was built upon certain assumptions (which will vary by user) and created with help from an online bandwidth calculator as well as information from the carrier sites.