The common wisdom is that you need to have a deficit of 3500 calories to lose a pound. So if you eat 500 less calories than you work off every day, you should lose a pound a week. Most advice also says not to go below 1200 calories a day. You can set a goal on your fitbit to help you monitor how you are doing. I find the dashboard reminder throughout the day to be helpful.
I agree with @jmkoon . It is not really about what you burn, but about what you burn vs. what you take in. FitBit has a tool under the Food Log where you can set the amount you want to lose per week. They then calculate what your "deficit" in calories should be per day to meet your goal.
My fitbit let me set a weight goal and it actually TELLS me how many calories I can consume in a day, taking how much I burn into account automatically. Check out: https://help.fitbit.com/customer/portal/articles/312294-how-do-i-track-my-calories-make-a-food-plan-...
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