I feel like I MUST be making a huge mistake here

StaciMarie1974

I have used a Fitbit for the past 6 months - and while it can't be 100% accurate, I've found it to be very useful as an activity tracker. Fitbit uses the established formulations for BMR and factors in activity. It can't tell intensity (not a HRM) but measures overall step-based movement. I officially set my goal for a 500 daily deficit, and generally try to have an extra 100-200 calorie deficit daily to help offset estimation errors. I lost 3.8 pounds in my last 4 week measuring period.

Better than someone assuming they're 'lightly active' and never knowing how much they actually move, to know if the setting is too high or too low.

WalkingAlong

As far as I can tell, fitibit is a glorified pedometer, NOT an HRM, so the calories she's tracking probably aren't accurate.

I think you should actually read up on it then, as it goes way beyond what most HRMs are capable of.

Everything I see just talks about counting steps, and how good it is at counting every step you take. I can't find anything that indicates it measures your heart rate at all. Counting steps doesn't tell you how many calories you've burned.

It takes the steps and uses a formula to calculate your calories burned. This of course is an estimate. A HRM does the same thing though. It takes your HR and uses a formula to give you your calories burned. It's an estimate. As far as accuracy, reading some tests people have done the FitBit seems to come pretty close to numbers HRMs give. Bottom line though either way, if it's 100% accuracy you want you're not going to get that anywhere.

Comparison of several devices: http://arijaycomet.com/2013/12/11/caloric-burn-comparing-heart-rate-monitors-to-activity-trackers/

From the results summary: "For purpose of my testing I’ve learned that the data I have been estimating using the Fitbit calculator is nearly identical to the data I logged using the Polar H7 HRM"

And you don't need accuracy. All you need is a baseline. Your results will tell you how accurate your estimates are and you can adjust your eating accordingly. Look at the Nike Fuelband. It doesn't even use calories as its metric, just a made up 'fuel points' value. Because it doesn't really matter. If I lose weight at 4000 fuel points a day and 2000 calories eaten, I'll lose more at 5000 and less at 3000 (and will lose more at 4000 points and 1800 calories/day than 2000/day).

I agree with the post above but I would add that technically the Fitbit doesn't use 'steps' themselves to estimate calorie burn. It uses the motion pattern detected from the accelerometer. You could rack up more calories from a lower step-count workout if it's got larger body motions and more impact, for example.