How accurate are fit bits?

Fit it’s aren’t accurate . Every person I know that has one adjust their calories.Being honest I wouldn’t go by one. I would go by your height and how active you are I have a Garmin Fitbit 2 and I know for a fact it’s not all accurate.
It all depends on what your doing. I know some construction plumbers and they walk a lot and do burn a lot while working and carry pipe .
And too are you working out besides job. If gym time you would definitely need to eat more healthy calories because of burning and building muscle.
Wish you all the best . I have some great friends and great support people . Men friends that hit the gym besides work . Good luck on your journey. 😁

Depends - if the Fitbit was slipping into using HR-based calorie burn because the HR was high enough - inflated calorie burn.
If your exercise is lifting and HR-based was used - inflated.

Daily stuff should be based on distance moved for more accuracy potential - and with that many steps - if stride length setting is off then distance is wrong - big change again.

Have you ever done a walk 1/2 to 1 mile known distance and confirmed Fitbit saw the right distance?
It should be at 2 mph - right in middle of potential ranges of pace you'd like have through the day, from grocery store shuffle to exercise level.
That allows the device best chance to dynamically adjust up or down based on actual pace then.

If no HR-based kicking in, and distance was accurate - I'd actually wager the Fitbit would be underestimated.
Because it gives all non-step time a base burn of sleeping level - and if you are standing and working - you are burning way more than that.

The numbers on reality method can be very good compared to a tweaked Fitbit - because your food logging could also be inaccurate by some many calories.
So in reality eating more than 2000.

But even with poor logging - these tools just let you hone in fast, and adjust perhaps for seasonal changes.
So while you may be eating an inaccurate and underestimated 2000 say and Fitbit says 4000 tweaked - you know if the Fitbit drops to 3500 during winter - you probably need to drop to 1500 eating to keep the loss.
And as approaching goal weight and need to slow the rate of loss, you know need to eat 2500 logged, whatever it may really be in reality.

Mine is very accurate. I don’t have one with HR monitoring.

I kept 3 months of data (calories in, calories out, daily weight) and my weight loss matched what my fit bit said I was burning.

If I’m not weighing all my food it looks like the Fitbit numbers are inflated, but it’s more that I’m not being accurate in measuring my portions and underestimating how much I’m eating.

Mine has been very accurate for me, but as others have said, I don't have HR on mine.

All of the manufacturers are very careful to state that the trackers are for "entertainment" purposes. They are not particularly accurate. Mine has HR and I've tested it against several things - measured distance, vs. Garmin running watch w/HR strap. and against HR done in the doctor's office. As to steps it is pretty darn accurate. As to HR it is very close. As to calorie burn it is a crap shoot. But then again MFP's numbers are wildly inaccurate as well and even with measuring and weighing a lot of this is still guesstimates at best. If your weight is moving in the direction you want, at the pace that you expect, keep doing whatever you are doing because it's working; adjust as needed. Good luck!