Hi Mommieoftwocuties, the allowances on Spark vs. fitbit are calculated differently.
Both use your profile stats (height, weight, age and gender) so make sure those are correct on both sites. Both use your current weight vs. your goal weight--so make sure that is the same on both sites. They do use different BMR formulas so tehre are likely to be differences--the formula Spark uses estimates my bmr at about 100 calories higher than the other bmr formulas.
Rate of loss: This is different between the two. On fitbit you choose what you want to lose each week (choices are .5 pound a week, 1 pound a week, 1.5 pounds a week, and 2 pounds a week). Choose the appropriate one, keep in mind fitbit doesn't have a minimum allowance, but I believe Spark might. So choose one that still allows you enough calories (I think probably 1200 r more--I don't know Spark's minimum by 1200 is a common minimum). Now after that is set up, fitbit will predict the date that you might reach your goal if you lose according to that schedule. Note that number. Spark's rate of loss is based on your current weight vs. your goal weight and what deadline you set to reach your goal--it will warn you if you choose to short a deadline. Enter in your fitbit prediction for when you will reach goal. This should make your daily deficit and rate of loss the same on both sites.
Activity: Spark assumes a specific activity level (they explain this somewhere), it is pretty sedentary. It only adjusts the allowance based on what you enter as your "weekly calorie burn goal from cardio" in the fitness tracker. Nothing changes in response to your activity other than giving you a "you are burning too many calories for your goal" message when you exceed whatever you enter as your goal. So there is a bit of an art to entering the right number in that goal for your actual activity. Fitbit is an all day activity tracker and is really more designed for the activity aspect than the diet aspect. So the allowance is based on your total calorie burn each day minus your intended deficit. So if you are set to lose one pound a week and burn 2,000 calories your allowance would be around 1500 calories.
The size of the range: Fitbit counts you as within goal if you eat within 50 calories of your goal so the range is smaller than Sparks. I have found if the lower end of my Spark range matches my fitbit goal when I am sedentary it works out okay. I just eat towards the upper end on very active days. For me the Spark range is a little too big without guidance on how to use it. Before I had a fitbit I would eat within range (usually at the upper end) and either maintain my weight or lose very slowly like 1-2 pounds a month (I also didn't have a brisk enough deadline set so I guess I was actually set for a pretty slow rate of loss without realizing it). But I suppose we had opposite problems. Fitbit doesn't give me that many extra calories beyond my bar, on a less active day my fitbit burn tends to be 1550-1600 calories and on a very active day (2 hours of exercise or activity, high step count, etc) see a burn of about 2100. My bmr that fitbit uses is about 1260 or so so t is only adding 300-800 to my calorie burn for activity. Of course calorie burn is very dependent on your personal stats it is really hard to compare between different people. Mine lines up with the Spark range pretty close if my settings match well enough and if I subtract my bmr for all my fitness minutes and if I enter the right number in my Spark weekly calorie burn goal.
Sorry that was kind of long, but I noticed several ways the Spark and Fitbit diet plans are different. They are not incompatible though if you match your settings.
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