How is calories burned during exercise worked out?

[Fuzzybojo]

I'm curious as to how the fitbit calculate calories burned during exercise. I thought it used heart rate, but I'm not so sure anymore based on some of my results. Here's the two sessions in question that have me wondering.

For context, the first one is from when I just started the gym, I'm going 5 days a week. Both are from an exercise bike where I've started the Spinning exercise as I began to pedal and stopped it when I got off.

26th August: 40 min, 151 avg bpm, 95% cardio, 5% peak and 595 calories burned.

5th September: 40 min, 153 avg bpm, 92% cardio, 8% peak and 580 calories burned.

As you can see, I expended more effort today than I did on my first session, yet I burned less calories. Common sense says I'm getting fitter so I have to push harder to burn the same amount, but how does the fitbit track that?

Many thanks.

[NellyG]

Hi @Fuzzybojo I dont know the intricacies of how calories are calculated, except that the higher the Heart rate in general the more calories burned. But to me your two days exercise look so similar that the differences seem negligable. You are not going to get absolute accuracy from a wrist worn fitness tracker, but you will get a close enough number in most cases. To me your stats look pretty good.

[SmyrnaR]

Yesterday I rode my stationary bike for 35 minutes with an average heart rate of 162 bpm and it calculated my caloric expenditure at 440 calories. 5 min cardio, 31 min peak

Today I rode for 47 minutes with an average HR of 182 bpm but only burned 192 calories. 3 min cardio, 44 min peak

whats going on?