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I use Kinetic to measure my regular excercise routine (walking). It calculates / estimates the amount of calories burned during that period.

I am wondering how accurate (generally speaking) these sort of calculations are? Are calories easy to calculate with tasks such as walking and I should trust it fairly well? Or are they usually bad enough that I should allow for about 50% allowance each way?

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All devices that display a "calories burned" number are showing estimates based on mathematical formulas. The formulas have been calibrated by gathering data from real people, but there are a lot of variables that affect how many calories you burn, and it's impossible to take them all into account. However, the more variables your program measures, the more likely it is to be accurate.

One very important variable is your weight. Does the device or app you're using ask you to input your weight? If not, it is likely highly inaccurate, since the calorie cost of exercise will vary significantly depending on a person's size. Does it count your steps? That will improve its accuracy. Does it take speed/distance into account, possibly using GPS? That will improve its accuracy. Does it know (again possibly using GPS data) if you're going up/down hill? That will improve its accuracy. Are you wearing a heart rate monitor, and is the device getting data from that? that will improve its accuracy.

The more variables the device has available to it, the better it can guess. The more unknowns there are, the more the numbers may skew toward some "average" or "typical" person's numbers, which may be very different from yours.

Even in the best circumstances, the estimates will likely be off. For example, in this article from Good Morning America/ABC News, a study conducted by The University of California is discussed where 4 different exercise machines calorie claims were compared with VO2 tests (a measure of calories burned by examining the amount of oxygen being consumed by a person's breathing). These are the results they found:

On average, the machines overestimated by 19 percent and the watches overestimated by 28 percent. Here's the breakdown: Treadmill: Overestimated calories burnt by 13 percent. Stationary Bike: Overestimated calories burnt by 7 percent. Stair Climber: Overestimated calories burnt by 12 percent. Elliptical: overestimated calories burnt by 42 percent.

This About.com article talks a bit more about the difficulty of estimating calories on a treadmill, and suggests that machines tend to overestimate. Here's another article from CNN. I don't know how similar Kinetic is to any of these machines, but it seems to me to be most like the "fitness watch" class of device. I searched for a study on the calorie-counting accuracy of pedometers, but didn't find anything authoritative.

In a nutshell, the calorie numbers given by machines are estimates that be can off, especially if the machine isn't tracking many variables. If the device or app is taking into account distance, weight, incline, heart rate, and other variables, then it's probably as good of an estimate as you can reasonably get. But still, you should recognize that it would not be unusual for your device to be off by 20% or so, depending on the circumstances.

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Thank you for taking the time to give such detailed feedback, I really appreciate it. That's interesting that most devices overestimate ... I would have hoped that they would underestimate and make me work harder to acheive my goals instead of making me feel good (seeing the numbers) but then wondering why my body isnt seeing the same results. Thank you again. –  Nippysaurus Sep 16 '11 at 0:23
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it's worth noting that even a VO2 measurement is only an approximation: indirect measurement. The only way to REALLY measure calories burned is with direct calorimetry - a type of extremely specialised device, indeed a whole room-like thing, that measures the heat output in to the whole "room". –  Joe Blow Sep 18 '11 at 20:36

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