What is considered net income? The law is located at 750 ILCS 5/505.
Child support is calculated based upon net income from all sources.
(3) "Net income" is defined as the total of all income from all sources, minus the following deductions:
(a) Federal income tax (properly calculated withholding or estimated payments);
(b) State income tax (properly calculated withholding or estimated payments);
(c) Social Security (FICA payments);
(d) Mandatory retirement contributions required by law or as a condition of employment;
(e) Union dues;
(f) Dependent and individual health/hospitalization insurance premiums and life insurance premiums for life insurance ordered by the court.
(g) Prior obligations of support or maintenance actually paid pursuant to a court order;
(h) Expenditures for repayment of debts that represent reasonable and necessary expenses for the production of income, etc.
SEE statute for more detail
What if both parents spend equal time with the children?
The residential or custodial parent is entitled to receive child support for the benefit of the children. Most courts prefer to stick to guideline child support, even when the non-residential parent spends a significant amount of time with the children. There are exceptions however. One example is when the parties are able to co-parent successfully especially if each parent spends about equal time with the children. In such a case, some courts will allow support to be calculated as if each parent pays to the other parent, and the difference is actually paid to the parent who would be responsible for the lesser amount. Another example is when the obligated parent does not make enough money to handle regular day-to-day expenses and a downward deviation from guidelines is justified.
It is always best to have the assistance of a seasoned family law practitioner when child support is an issue.
Additional resources provided by the author
The law, known as the Illinois Marriage and Dissolution of Marriage Act (IMDMA) concerning child support and other family matters can be read at 750 ILCS 5/.