For purposes of determining which market an employer belongs in the Affordable Care Act defines a small employer as one “employed an average of at least 1 but not more than 100 employees on business days during the preceding calendar year and who employs at least 1 employee on the first day of the plan year.”
For plan years before January 1, 2016, states can define small employers as those having 50 or fewer employees.
Under this definition part time employees are given the the same weight as full time employees.
Suppose an employer has 40 full time employees plus another 40 part time employees. All of these employees work each day the business is open. In this case this employer would have 80 employees.
I live in Tennessee where the state has chosen to define a small employer as having 50 or fewer employees. So in my state this employer would be a large employer.
Please Note: This is not the method that is used to determine whether an employer is supposed to offer health insurance to its employees. For these you need to determine the number of full time equivalent employees. This means that part time employees are not given equal weight to full time employees.