The law defines large employers as those businesses that employ an average of at least 100 full-time equivalent (FTE) employees in 2015, and goes down to 50 in 2016. Large employers are subject to the employer mandate and must offer coverage to full-time employees or face possible penalties.
First, you must determine ownership and what businesses must be counted in the FTE calculation. The IRS will apply its "common control" standard. So companies that have a common owner or are otherwise related generally are combined together for the purposes of determining whether or not they employ at least 50 full-time equivalent employees.
For example, if you own at least 50% of a restaurant, 50% of a remodeling company and your wife owns at least 50% of a gift shop, those three businesses must be considered together as you calculate your FTEs. Tax Code definition of employer
The threshold of 50 or more full-time-equivalent employees is determined by the following formula, which you would calculate on a monthly basis:
Part-time employees are counted only in determining whether an employer meets the small-business exemption from coverage under the law. In no case do employers have to provide health care coverage for part-time employees or pay penalties for part-time employees.
When do I do this calculation to determine if I am subject to the employer mandate?
For employers who are on the edge of the 50-FTE threshold, the IRS allows employers to measure their workforce for any six consecutive months in 2014 --- rather than the full 12 months -- to determine if they had 200 or more full-time-equivalent employees during that period. So, for example, an employer could use the period from Jan. 1, 2014, through June 30, 2014, and then have six months to analyze the results, determine whether it needs to offer a plan, and, if so, choose and establish a plan.