Calorie Confusion

[DaveBR]

According to most BMR calculators I should burn about 1,833 calories a day just by existing. I am male, 6'0" tall and 215 lbs. Fitbit records 6.4 calories per 5 minute increment for inactivity. That's 1,834 for 24 hours. Great!

However, if I create a day in the past - before I have a Fitbit record, it shows 6.4 calories per 5 minute increment on the graph, but then it records 2,605 calories for the day by number. That's 762 extra calories. Why is the total different?

Furthermore, if I add a mythical activity for this mythical day - say an hour of biking at 800 calories - the calorie total DROPS to 2561. Now that is actually close to 1834 + 800 - (6.4 x 12,) the calories I would have burned for doing nothing. It seems accurate.

Am I to assume the mythical day, with no data, is just an aboration, and that once there is data it's all good again? Has anybody else noticed this?

Thanks.

[Dawn1129]

 Maybe helpful, or maybe not, but it could be that the fitbit adds in your level of activity (as you have it entered for your settings) as an estimate to the BMR for the mythical day.  When you add in the actual extra activity for the day, it may be reverting to your input instead of it's estimate.  BMR is if you were sitting in a bed on life support, (or so I've been told) so it's logical that it would estimate the 2605 considering in a sedentary or lightly active lifestyle. 

[gerrymcd]

I think these kind of numbers can only ever be a rough approximation at best.  To get an exact value would require spending some time in a physiology lab where they measured your mass/fat/muscle accurately, together with your respiration/metabolism.  The best that websites can do is use data from 'average' people of the same height and weight -- together with real data from your fitbit/heart rate monitor, etc.   The combination of 'real' data with 'average' values can lead to some discrepencies, especially if you are more/less fit than average, have more/less muscle mass, etc.