This is a good rule of thumb to follow thanks to the International Health Racket and Sports Association.  I like to work the calculation like this:

Membership capacity = NCD x TNM

Where:

Number of cycles per day or NCD = total hours of operation / average length of member visit

Total number of members club can accommodate or TNM = person on cardio machine + person on aerobics floor + person at each locker room + person anywhere in club space…..

Description by IHRSA

6. CALCULATION: How can a club calculate the membership capacity of it’s fitness center?

  1. Determine the total number of members the club can accommodate at one time by counting all strength training equipment, cardiovascular equipment, number of spaces on aerobics floor.For Example: 1 member per every cardio unit, one member per every aerobics space, one member per every two pieces of strength training equipment
  2. The sum of those three ‘fractions of usage’ is the maximum number of members ‘in usage’ that the fitness center can serve at one time. Keeping in mind that usage of locker rooms also needs to be accounted for.
  3. Add in usage and locker room figures to determine the total number of members the club can accommodate at one time.
  4. Divide hours of operation by the length of the average visit to determine the number of membership cycles per day. Multiply that number by the total number of members the club can accommodate at one time to determine base membership capacityNOTE: One needs to account for low use members as well as seasonal members
  5. The final represents the club’s capacity.

EXAMPLE – Jo’s SLIM GYM

12 cycles per day = 12hrs  operating hours / 1hr average length per member visit

20 members club can accommodate = 18 aerobics floor + 2 private sessions

12 x 20 = 240 members

Full article can be found at the following link

IHRSA calculate membership capacity