Exactly how many calories do you burn in a day? For the health-conscious (or just data-obsessed), this isn't as easy a question to answer as it seems. There are millions of calorie burn calculators out there, but they are all estimations. Do you work entirely seated during the day, or semi-seated? Are your walks to and from lunch fast enough to count as "brisk," or are they really just strolls? How hard are you really working when you run? The estimates might come close, but they vary so wildly that it's hard to really know what to believe. Do I really burn 1,750 calories just by my body existing for a day, and 2,500 calories by actually being awake and doing stuff?

These are the types of questions that sparked the development of a new type of fitness gadget—the kind that latches onto your body in an attempt to measure your real calorie burn. These devices do this by measuring your body temperature, the rate at which heat is leaving your body, sweat levels, pulse, and body motion, among other things.

The most famous of these is the BodyBugg, used on the popular weight loss show The Biggest Loser, but there are others just like it—in particular, the BodyMedia FIT, which is essentially the same product as the BodyBugg but under different branding. (In fact, a BodyMedia spokesperson confirmed to us that BodyBugg is actually owned and developed by BodyMedia.) We were given the opportunity to try out a BodyMedia unit, and found that it gave us all the data we ever wanted to know, and then some.

What you get and how you use it

The standard BodyMedia FIT "package" is the armband for $190, plus the online activity manager that requires a monthly subscription (this ranges from $7 per month if you pay for 12 months at a time, up to $13 per month on a month-to-month basis). There's also the armband plus a wireless display, which is a little device that shows you in real time what your armband is reading—these two things cost $260 together, plus the same online activity manager subscription fees. We were given review units for the armband and display:

The "armband" portion is actually a USB-capable device that is locked into a stretchy band that goes around your arm. The unit itself can pop out of its holder so you can plug it into your computer via USB to sync with the activity manager on either a Mac or Windows machine. BodyMedia claims that it can monitor 9,000 different elements of your body in order to come up with a calorie burn estimate, as well as a number of other metrics.

As mentioned above, the separate display unit only serves to show you the data that your armband is collecting and whether or not you have met your goals for the day—the system does not require the use of the display if you don't want to pay extra for it, but we found it very useful when measuring the calorie burn of specific activities.

When you're being tracked by the armband, the display can show you things like your total calorie burn for the day so far, your total from yesterday, and the number of steps you have taken so far today, There's also a "trip" mode, which works sort of like a trip meter on your car that allows you to reset the number of miles for a specific trip—when you reset the trip mode on your display, it will go to zero (calories, steps, etc.) and allow you to see how many calories you burn from the time you reset it.

The armband is meant to strap to your arm for every waking and sleeping moment of the day—the only time you shouldn't wear it is when you're in the shower or swimming, as the device isn't waterproof. It isn't terribly bulky and can easily hide underneath a shirt with loose sleeves. The device straps to your tricep area, and the left arm is recommended, although you could use either arm. Since it can also measure your sleep cycle, you're supposed to wear it while sleeping too, although if you're a side-sleeper, you may find yourself feeling a little like the Princess and the Pea if the unit is strapped to your preferred sleeping side.