What's eating your phone's data allowance?

— -- Question. Could all the spam I'm getting explain why I use up half of my phone's data cap in a couple of weeks?

Answer: Sorry, probably not. Most e-mail takes up little bandwidth, and spam — typically, several lines of text for each junk e-mail — uses even less.

Most mobile Web browsing also doesn't do much to eat up a bandwidth quota. To get a grasp of what apps can get you into trouble, employ a few simple data-usage tools.

Apple includes a bandwidth meter on the iPhone (open the Settings app, tap its General heading, then Network) and the 3G and 4G versions of the iPad (Settings, then Cellular Data). This will report the total data consumed by the device; third-party apps can provide the same info for WiFi-only iPads and the iPod touch.

On Google's Android, the Ice Cream Sandwich version now shipping on a few devices offers a more nuanced data-usage gauge that breaks down the bill on an app-by-app basis. On earlier versions, other firms' software has to suffice; I've run the free NetCounter.

Years of spot checks with these tools have convinced me that Web radio — sometimes labeled a bandwidth hog— shouldn't be a problem either. For example, I've clocked Pandora's Web-radio app using no more than 30 to 40 megabytes per hour of listening.

Web video, however, can get the meter spinning — especially on devices with larger screens. On an Android phone, an hour of Netflix viewing ate up almost 180 MB. But on an iPad 2, an hour of Netflix accounted for about 670 MB (though that figure may include background activity such as checking e-mail). On a new Android tablet, the same 60 minutes of the same movie chewed through 656 MB.

I've heard of even worse examples —Houston Chronicle tech columnist Dwight Silverman recently tweeted that watching two 30-minute episodes of The Big C in Showtime's app on a new iPad ate up a full one gigabyte of Verizon LTE data.

Unusual situations can yield odd results, however. When I took a Samsung Galaxy Nexus to the SXSW Interactive conference in Austin and cruelly overused it, its data-usage gauge reported that my frequent Web use topped the leaderboard, at 117 MB for that week. Facebook followed at 56 MB, then Google's Play Store (what used to be called the Android Market) at 47 MB. That's a lot of News Feed scanning and software updates.

The easiest way to stay out of trouble with bandwidth caps, however, is not to worry about particular apps but to use Wi-Fi instead of 3G or 4G whenever possible.

In a follow-up e-mail, the reader who sent this query suggested that his frequent lookups of satellite photos and weather radar could have been at fault. I don't know that he's right, but those theories make more sense than an overload of junk mail — not that you don't have plenty of other reasons to be annoyed at that.

Tip: Battery-saving black backgrounds on some phones

The abbreviations used to describe the screens on mobile devices usually invite yawns — if people take note of them at all. But one particular kind of display, OLED, allows for a special power-saving trick.