Calling all fitbit-ers!

I don't use a fitbit anymore because my One's battery finally gave out and any other version grossly overestimated my exercise calories but I have, and always will, eat every last calorie allotted to me. 😀 One, I earned them. Two, I was already in a deficit when in weight loss mode and not eating the exercise calories back just made that deficit bigger and potentially harmful because of undereating. Three, more food! Four, I really like food. Five, ok I exercise FOR food. I'm not gonna not eat it. 😀

Some good advice around here is eat 50-75% of your exercise calories back and reevaluate if you're hitting your weight loss/gain/maintenance goals in about 6 weeks and adjust from there.

There are two questions implied here, I think:

1. Should one eat back their exercise calories or not? and
2. Will Fitbit's estimate of exercise calories be accurate?

Number 2 first: Any well-designed, well-programmed device, which Fitbits tend to be, that is operating correctly, is going to give you a calorie estimate that's essentially an estimate based on research on people demographically similar to you.

Implication: If you're close to average, the estimate will be close to accurate. By definition, most people are close to average. A few people may be a little off (high or low) and a very rare few surprisingly far off (still in either direction). After you let a new device "learn" you (couple of weeks, maybe?), and follow its estimates for say 4-6 weeks, you'll have a good idea how close to average you are, thus how close the estimate is for you.

Things that may help accuracy: If it uses heart rate, making sure it knows your true maximum heart rate (if you have a better idea of that than an age estimate), or VO2max (ditto), if it lets you input your own value. (The age estimates are significantly off for a fair fraction of people. It's more about genetics than anything else.) Testing your stride length for average daily walking speed and adjusting that if the device permits. (Other people can give you better how-to on that than I can; I don't care much about steps-based calories.)

Back at the general question #1 of eating back exercise calories:

If you got your calorie goal from MFP, and correctly set activity level in your profile based on activity before intentional exercise (so based on things like job and home chores), yes, you should eat back a reasonable estimate of your exercise calories, generically speaking.

If you have a slow loss-rate goal (say less than 0.5% of current weight per week), and don't do a lot of exercise (maybe low hundreds calories 2-3 times a week), then it's probably safe to let the exercise increase your calorie deficit (make you lose a little faster). (Do monitor actual results, though!)

If you have an aggressive loss-rate goal (couple of pounds a week, 1% or more of current weight per week, something like that), and do a lot of exercise (few hundred calories or more most days), then not eating back at least a fair chunk of those calories increases health risks, can sap your energy so be counterproductive, and generally not be a good idea.

In between those extremes? It's a question of how much health risk you welcome into your life, how much other physical/psychological stress (besides your calorie deficit) is in your life (because stress is cumulative across sources), and things like that.

Some calorie estimation calculators (like many of them outside MFP) average in exercise calories to one's calorie goal. MFP intends for you to estimate them separate from daily life calories. It's important to understand which method you're using, and how to use it. Undereating can be counterproductive, even dangerous. Overeating obviously results in slow/no weight loss. You want to be in the sensible middle, IMO.

Any sane method takes exercise calories into account somehow. If it doesn't, that amounts to giving a couch-sitting TV-watcher the same calorie goal as a iron man triathlon training gym rat, which clearly isn't sane, y'know?

Bottom line TL;DR: Eat back some or all of the Fitbit calories for 4-6 weeks, see what your results are.

If you are worried about the accuracy of the calorie using the fitbit then you could use another program such as MapmyRun that ties to MFP. When I log a training in Mapmyrun it drops that data into MFP. I have found that the MFP activities are limited and this other one is not.
I usually do not eat all of my calories back part b/c I want to "enjoy" the reduced calorie intake and I think it helps lose weight (at least for me).

Exercise should be accounted for one way or the other to avoid having too large of a deficit and harming your body. Many calculators utilize the TDEE (Total Daily Energy Expenditure) method for which you include an estimate of your exercise and daily life activity in your activity level and you get a calorie target for weight loss that includes all of that activity.

MFP utilizes the NEAT (Non Exercise Activity Thermogenesis) method for which exercise is NOT accounted for in your activity level. Deliberate exercise is accounted for after the fact when you do it and log it thereby getting additional calories to consume. With TDEE those calories would have already been included in your base so eating back those calories would be double dipping...with TDEE you would have a higher base calorie allotment.

Is 600 calories an overestimate? Possibly...but you also certainly burned more than zero. I really don't understand the logic of "this might be an overestimate so I won't count anything at all". Part of this is learning on the fly. Making your best possible judgement on you calorie expenditure as none of this is some exact science...but failure to account for exercise, particularly if it's a lot of exercise, can lead to a deficit that is too large to be healthy.

If your calorie target is 1200 without exercise and you indeed burned 600 calories with exercise then your net calorie intake is 600 calories...meaning it is the equivalent of only feeding your body 600 calories which is a starvation diet. Proper nutrition doesn't only mean eating your fruits and veg and getting those essential vitamins and minerals...It also means giving your body an appropriate amount of energy (calories) to function. You burn a *kitten* ton of calories merely existing and doing nothing else.