When doing a BBL - how much of the injected fat usually stays? To my understanding its only 1/3? Is that about right?

Hello dear!!
Thanks for the question and provided information as well.
After a BBL, about 80% of the transferred fat cells will survive, but this should last permanently, with exercise and a clean diet. Lifting, squats and lunges after a BBL will only help increase muscle bulk and definition and achieve a more projected buttocks.
If you have any concerns, I suggest you follow up close with your plastic surgeon.
Good luck :)

They key to graft survival is not jamming as much fat as you can into the buttocks; it takes
meticulous injection technique. If done carefully in small aliquots, you can
expect at least 60% survival. There is a maximum amount one can inject in one
session as the amount of viable graft bed decreases as more graft is injected.
Make sure you find an experienced surgeon who knows the fundamentals of fat
transfer . . .

Most surgeons quote around 60% fat graft survival rate. There are studies that suggest that graft survival decreases with increased pressure common with large volume transfers. Therefore, I prefer to process the fat in a way that decreases the total volume transferred so as to decrease the pressure within the buttocks after the transfer and thus increase fat graft survival. Don't get hung up on the volume of fat transferred as this varies based on the way the fat is transferred, as most of the time all of the fat harvested is used to change your shape if necessary and augment your buttocks. Hope this helps and best wishes.