How does Fitbit charge HR calculate calories burned?

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ahoy_m8
ahoy_m8 Posts: 3,053 Member
I've searched Fitbit forums, and people ask this question, but the answers are vague and don't answer the question. The answers say it starts with BMR. Ok, so the formula is BMR+something. Does anyone know the formula?

The default stride is wrong for me. It underestimated my mileage. When I log exercise (vs the automatic entry), it adds mileage and steps but lowers my calories burned. That tells me the formula employs variables in addition to heart rate. Just curious.

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  • armydreamers
    armydreamers Posts: 175 Member
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    Good question. Following!
  • CharlieBeansmomTracey
    CharlieBeansmomTracey Posts: 7,682 Member
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    do a search here on the forums and there are many topics on this. some may make it easier to understand than others. hope that helps
  • MeanderingMammal
    MeanderingMammal Posts: 7,866 Member
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    ahoy_m8 wrote: »
    I've searched Fitbit forums, and people ask this question, but the answers are vague and don't answer the question. The answers say it starts with BMR. Ok, so the formula is BMR+something. Does anyone know the formula?

    The default stride is wrong for me. It underestimated my mileage. When I log exercise (vs the automatic entry), it adds mileage and steps but lowers my calories burned. That tells me the formula employs variables in addition to heart rate. Just curious.

    So it estimates in two different ways

    Since most background activity can be assumed to be step related it makes an assessment of distance covered, based on steps counted. As you highlight changing the step length to more accurately reflect your actuals will help the accuracy there. It adds that to BMR.

    When it thinks you're doing deliberate exercise it makes the assumption that you're doing steady state CV work in the aerobic range and works from there. That assumes that it can work out your VO2Max and Resting Heart Rate. The latter is simple, the former not so much.

    So as long as you're doing aerobic range steady state, like running or cycling, then it shouldn't be too inaccurate. As soon as you deviate from there, it's likely to be wrong.
  • mmsilvia
    mmsilvia Posts: 459 Member
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    I belive you can adjust your stride on their website
  • ahoy_m8
    ahoy_m8 Posts: 3,053 Member
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    ahoy_m8 wrote: »
    I've searched Fitbit forums, and people ask this question, but the answers are vague and don't answer the question. The answers say it starts with BMR. Ok, so the formula is BMR+something. Does anyone know the formula?

    The default stride is wrong for me. It underestimated my mileage. When I log exercise (vs the automatic entry), it adds mileage and steps but lowers my calories burned. That tells me the formula employs variables in addition to heart rate. Just curious.

    So it estimates in two different ways

    Since most background activity can be assumed to be step related it makes an assessment of distance covered, based on steps counted. As you highlight changing the step length to more accurately reflect your actuals will help the accuracy there. It adds that to BMR.

    When it thinks you're doing deliberate exercise it makes the assumption that you're doing steady state CV work in the aerobic range and works from there. That assumes that it can work out your VO2Max and Resting Heart Rate. The latter is simple, the former not so much.

    So as long as you're doing aerobic range steady state, like running or cycling, then it shouldn't be too inaccurate. As soon as you deviate from there, it's likely to be wrong.

    Thanks a bunch for this. I'm curious about VO2max, too. I had it measured once in my 20's and it was low for my age/condition. Was running 10 mi/day at that time. Im curious also about what resting heart rate really means. The average Fitbit calculates is 20 bpm higher than when I'm actually resting, like sitting for a while or asleep. I'm a little confused by that.

    I do sprint intervals (8 of them over a 4mi course) 3x/wk, and I'd like to see how my hr rises and recovers. The 5-min hr interval doesn't provide a granular/accurate picture. Intervals seem to confuse Fitbit, or rather, ar not a good fit to its algorithms. Just learned today about setting the exercise mode, so I'll try that tomorrow. I think that will help. I log it as walking, because that's what I'm doing for most of that 45min. Incidentally, sprint intervals are more effort than just walking, but Fitbit gives me more calories for walking 4mi in 50 min. Go figure. Still learning.
  • ahoy_m8
    ahoy_m8 Posts: 3,053 Member
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    mmsilvia wrote: »
    I belive you can adjust your stride on their website

    Yes, I believe so, too, but have not yet measured my stride. Rather than going to a track and counting, I think I'll nudge the default by a %estimated by how off its previous estimates have been. That will be closer, anyway. I'm still a little confused how to account for differences in my stride when I'm walking most of the day in high heels vs. walking 12-min miles for exercise vs. running. I realized I don't mind if mileage is off, because I do know what my exercise mileage is. HR and calories burned would be new information for me, though.