Child Support in Illinois is required to be determined in both Divorce and Parentage cases. The state law regarding how child support is calculated can be read at 750 ILCS 5/505. Since 2016 child support is determined based upon both parents income and the amount of parenting time each parent has. A significant factor is whether or not both parents have at least 145 court Ordered overnights. If one parent has less then 145 overnights such parent will be paying child support to the other at the highest rate.
There is no longer a simple child support formula based upon percentages as the case prior to 2016. The below image is the Illinois Supreme Court child support form called an Order for Support. As you can see there are multiple determinations which when combined produce a child support payment owed from one parent to the other. It is even possible in a case where parents have shared or 50/50 parenting time that one parent may have to pay child support because of the differences of income.