For the same type of class,the changes tend to be rather small (and the pay is usually not very much to begin with). At the club where I teach, there was an increase from $22.50 to $25 which is a little more than 10 %.
I agree with Sue D’Alonzo. If you have an instructor that packs in the students, give them some type of per student pay out. Or quarterly bonus. And don’t forget to express your appreciation of their hard work.
For less popular instructors you should still give them a pay increase that is not too small to be appreciated. Come up with an amount that you feel would be reasonable for the year and divide it by their annual average hours to increase their hourly rate by. Be sure to thank them for their hard work as well and point out the annual dollar amount that their raise will translate into for them.
I hope your instructors appreciate their increases. Many fitness businesses never give raises and have a high turnover as a result. And I hope appreciate loyal instructors, even the ones that are “difficult”. They make your life easier than hiring new people over and over.
It depends on how long this instructor has been with you (or the company). Certifications, class attendance, experience, and productivity should be taken into consideration. If you think this instructor does a great job and don’t want to lose him/her, then a 10%-15% increase would be a fair number.