Garmin Forerunner 205 algorithm question

I’ve been using one since Christmas. It’s a runner’s watch with a bunch of features, the principal one being a GPS unit that tracks your speed, calories burned, elevation changes, etc. You can download your data to your computer and track all sorts of running minutiae (I don’t).

Is anyone familiar with how the algorithm for determining calories burned for these devices works? I assumed at first it was pretty simple, and just used ongoing pace, distance, and my weight. But I’ve discovered significant differences when I run the same distance, roughly the same time. For example, yesterday morning I jogged 20 minutes at an 8:34-minute mile pace (any real runners, don’t giggle). This morning I jogged 20 minutes, 8:33 pace–and there was about a hundred calorie difference.

I’ve check (a little) and can’t find how the algorithm works. I’m sure Garmin considers the details proprietary, but I’m just trying to get a sense of how it might work. Does it factor in temperature, elevation change? Is it measuring heart rate somehow? I also haven’t ruled out the possibility that it’s just imprecise.

Can anyone shed any light on how this (or similar devices) works?

Might you have been running a different route? My first thought was that it might take hills and altitude changes into account.

Sometimes different, but I have found it occurs even with one of my standby routes.

It takes altitude into account - and is pretty inaccurate at measuring altitude, as are most GPS devices unless major altitude changes are involved and theres good contact with multiple satellites.

Its also awfully optimistic calorie wise, and there have been updates to try and make it more accurate, so make sure you have the latest version with the updater software.

If you use Sporttracks (google), you can upload your data to that and it uses an alternative algorithm thats considered to be more accurate, and also more cautious about altitude changes. It also lets you see on google earth where you’ve been running, its a fantastic free program.

Sorry bit pluggy, but I was quite taken by it.


Otara, thanks for the tip.

WAG: It also tracks your heart rate, and your body uses different fuels based on your heart rate, correct? Perhaps that’s the answer.

Incidentally, if you haven’t already, find out your 5 heart rate zones and program them into your Forerunner. You’ll probably have to go somewhere and get tested. But I find knowing the zones and monitoring which zone I’m in is easier than monitoring the actual heart rate.

And finally, did you know you can make a google map from your Forerunner? It’s true, but I can’t remember the exact steps. I know some cyclists use it to generate cue sheets for rides.

The 205 doesnt measure heart rate, only the 305 does. The program I cited lets you print out jpegs of your run using google earth, as well as street maps and a few other options.


As Otara said, inaccuracies in measuring altitude are the most likely culprit. If you have a less-than perfect satellite signal, the altitude can easily fluctuate by 100ft or more even on the level. If the unit thinks you’ve been running up and down lots of hills it will presumably overestimate the calorie burn.

Makes sense. The Garmin 205 does let you program something as a standard route. Presumably then it’s smart enough not to think the earth has shifted since the last run (or that I’m running underground).