This depends on the gym. There are so many factors one must take in consideration before answering this question. After spending many years in a gym environment, in my opinion I believe the following factors are important to consider when owning a gym (which I’m sure you might already have thought of):
-Location and accessibility
-Type of equipment provided
-Condition of equipment
-Classes available to the members
-Customer service satisfaction
-Type and price of membership
-Marketing and advertising provided/available
-% of member retention
-Reviews and testimonials
-Image the gym tries to promote
-Level and experience of trainers and instructors
-Funds available for rainy days and continuous evolution to keep up with the competition and new trends
-Number of years being providing services
I’m sure there are probably many more factors out there which I can’t think of right now. In my opinion there is no magic formula out there that can give an accurate number of membership retention and expectation. If your gym and management team can stay on top of their game and makes sure all of the above factors are met in a positive way, then your gym will be successful (no matter what the yearly membership expectation is). If not, then you can do the math :-).
I’m not sure if this answer your question, but I hope it helps. I’m sure there is a formula out there, but to me having figured out these factors will make the gym (as any other type of business as well) a success. Even if a gym manages to increase the number of members in a year, if their services provided are not as expected then they have failed.