How to measure calories burned? How can I accurately measure the calories burned while using my bicycle?  My bike computer tells me how many calories I have burned.  But it has no cadence meter. The only measurements it has are my weight, distance, instantenous speed, and time (and all information it can derive from that).  Using the GPS on my phone, and the software from Sports Tracker on the same route, I got wildly different results.  The information on sports tracker yielded about 1.5 times more calories burned then on my bike computer.  Now, the GPS has a little more information, like altitude, but the ride was mostly flat. To add to that, there was an elevation drop over the entire route, so I don't know why it would say I burned more calories.  Do these devices use standard formulas, or do they just make stuff up?

Is it even possible to accurately measure the calories burned?  It seems that even if I had a cadence meter, there would still be a lot of variables.  Wind speed would make a big difference.  If you pedal with the wind at your back on a fixed gear, going 30 km/h, you are going to use a lot less energy pedaling the same bike at the same speed if you are going into the wind.  Note that in both these cases your cadence would be the same, but your muscles would be working much harder in the case where you were working against the wind.   Same could be said for things like hills, although this could be done with altitude, although I'm not sure how accurate you could get. Road conditions could also make a big difference, not to even get into the riding characteristics of the bike.
Yes, it's possible to measure calories burned. To do so accurately you need some sort of power measurement device. 

More accurate and more portable are the mechanical systems that attach to your bike - PowerTap is one example. These measure torque and speed at some point and work out power from that, often with corrections for various factors (input vs output power being the most obvious).

Less accurate but possibly more useful to you for training are the systems that attempt to work out the  biological power input. They work by measuring the difference between inhaled and exhaled oxygen or carbon dioxide, and generally use terms like VO2 Max. The apparatus for this is not even vaguely portable, so from those numbers they derive a relationship between your heart rate and your moving average power output (the response is not instantaneous), and on from there to your total energy output.

The cheap systems you see advertised will either measure bicycle speed and from that attempt to guess your power output, or measure your heart rate and guess likewise. Obviously the "what sort of bike do you have" and "what sort of hreat rate response do you have" questions are at best vague approximations to the truth and will vary over time. But the measurements are dramatically cheaper to make so they're consequently more popular.

A few years ago a friend looked at various mechanical systems and decided that PowerTab was the only system that was usefully accurate for him. We did break a couple of the cheap systems while trying to improve their accuracy, but that was acceptable because as sold they were useless to us. The VO2 systems are the definitive training aid because they tell you more than just average power output, and the other outputs are actually more useful than power output. Mechanical systems are the easy, every day measurement that gives you your overall progress, or for us, lets you evaluate the vehicle performance as well (to some extent we don't care whether you are five seconds faster at the same power output because the vehicle is more efficent, it fits you better or you have got stronger, we just care that the combination is more efficient).