Varying calorie goals amongst websites

[HillySuze]

I'm at a loss to know what my calorie range should be. Fitbit tells me I should consume 1759 per day (am I supposed to eat back calories?). My Fitness Pal tells me 1330 (I know you're suppposed to eat back calories) and Spark People tells me I should have between 1240-1590 without eating back calories.

These are all set for the same weight and 1.5 lb weightloss per week. How can they all be so different and how do I know which one I should follow?

[slysam]

I know that is confusing! I have accounts on all three, what I observed...

All three sites use an estimate of what you would burn at rest (basal metabolism rate or BMR and sometimes called RMR or resting metabolism rate) based on your height, weighgt,a ge and gender. THere are a few different popular formulas to estimate BMR that vary from one another. In real life, metabolism is a little more complicated than your height, weight,a ge and gender and all are probably fairly accurate for some and not for others. 

Spark People uses a different formula than Fitbit or MyFitnesspal. It uses the Harris-Benedict formula which for my stats is the most generous. This formula assumes I burn about 130 calories more just existing (no activity at all) than the other popular formulas. It tends to give me the highest allowance and when my fitbit account was set up to add exercise my Spark allowance for weight loss would end up higher than my Fitbit calorie burn. I used Spark first, and didn't really lose weight following their allowance for me so I tend to think it estimates too high (for me). I manually change my Spark allowance to fit my Fitbit range (the low end what my allwoance would be on a sedentary day and the high end my allowance on an active day) and I have it set up not to add exercise calories. You can get it a little tighter by playing with the deadline in which you want to be by goal. The other sites do a weekly loss and this does a deadline for when you want to be at goal (which is a little harder to figure out). 

Fitbit...  Fitbit uses a slightly smaller BMR estimate than MFP (mine is 2 calories lower). Your fitbit burn ends up being your total Fitbit calorie burn minus whatever deficit you requested. So yes, you do have a higher allowance on active days than less active days. Your allowance updates everytime you sync your fitbit so it will change in response to your calorie burn. But the extra you eat is included in your allowance.  If set to maintain, your allowance would match your calorie burn by the end of the day.

My Fitness Pal, do you ahve fitbit and MFP linked? MFP uses the Mifflin BMR formula (and maybe Fitbit does as it is a very small difference). MFP has you chose an activity level and assumes what you likely burn based on that. It intends you to log exercise and eat those calories. If you ahve a Fitbit linked, you don't necessarily need to log exercise to MFP because MFP and Fitbit compare calorie burn and MFP will adjust your allowance if you burn more or less than it estimated you would. So both intend you to eat according to your activity level. Both intend you to eat 750 calories less than your total burn (for your goal). My MFP allowance usually ends up pretty close to Fitbit in total--that means your MFP starting net goal + MFP logged exercise + fitbit adjustment. It makes more sense if you look at completed days.