Practices vary, but the number matters more than the benchmark. Tracking how many clients leave with their next appointment booked, as a share of clients seen, and watching it monthly, tends to improve it. Most therapists have never measured it.
Practices vary enough that the benchmark matters less than the trend, and almost no therapist measures it at all.
How to calculate it: the number of clients who leave with their next appointment booked, divided by the number of clients seen, over a month.
That single number tells you more about the health of the practice than revenue does, because it determines how much of your calendar you have to refill from strangers each month. A practice with a high rate is stable and largely markets itself. A practice with a low one is running to stand still regardless of how many new clients arrive.
What to do with it:
Separate first-time clients from returning ones if you can. The rebooking rate on first visits is the number that reveals whether the initial experience is converting.