The 'calories burned' numbers can't be trusted unfortunately. When it tested it when I first got my Fitbit I found that there was a 700 or so calorie discrepancy per day, averaged over a week. My guess was and still is that as Fitbit uses heartrate to estimate calories burned it will overestimate for a person who is very unfit, as I was at the time, and perhaps underestimate for a person who is very fit. I was hoping some others would reply to the thread to help to confirm my guess but nobody did.
The thread includes an image of a spreadsheet I use to estimate my actual daily calorie surplus or deficit, which includes formulas. The idea is to use changes in body weight from week to week to indicate how many calories were burned last week relative to calories eaten. This information can be used to plan changes for the coming week - i.e. it will enable you to estimate how many more or less calories you would need to eat per day on average to hit your target weight loss or gain rate. The 'calories burned' figures in Fitbit are a useful way to compare exercise sessions with each other - if one bout of exercise shows that it burned double the amount of calories than another exercise then that is useful information - but the daily totals for calories burned aren't trustworthy in my experience.
https://community.fitbit.com/t5/Manage-Weight/How-accurate-are-your-Energy-Burned-numbers/m-p/551337...
