Child support in Illinois does not simply take a percentage of the paying parent’s paycheck. The state adopted a model that starts by combining both parents’ incomes together, then works backward to figure out each parent’s fair share.

The Income Shares Model Explained

Under 750 ILCS 5/505, Illinois uses the Income Shares Model, built on the idea that a child should receive the same proportion of parental income they would have received if both parents still lived in one household. This starting principle shapes every step of the calculation that follows.

Calculating Each Parent’s Net Income

The process begins with determining each parent’s net income, meaning total income from all sources minus specific allowed deductions such as federal and state taxes, Social Security, and Medicare contributions. Illinois law allows this to be calculated using either a standardized tax amount formula or an individualized one, depending on the parent’s situation.

Combining Incomes And Finding The Basic Obligation

Once each parent’s net income is determined, the two figures are added together to reach a combined net income. That combined number, along with the number of children involved, gets applied to Illinois’s Schedule of Basic Child Support Obligations to find the total amount the state estimates two parents in an intact household would spend on their children.

Dividing The Obligation Between Both Parents

Each parent’s individual share of the basic obligation is calculated based on their percentage of the combined net income. A parent contributing 60 percent of the combined income is generally responsible for 60 percent of the basic support obligation, which then gets adjusted based on the parenting time arrangement.

Additional Costs Layered On Top

The basic obligation from the schedule is rarely the entire picture. Flat Fee Divorce Solutions helps clients anticipate these add on costs early so the final number does not come as a surprise. Health insurance premiums for the child, work related childcare expenses, and other add on costs get divided between the parents according to their same income percentages, increasing the total amount owed beyond the base calculation.

What Documentation The Calculation Actually Requires

Because the entire formula runs on accurate income figures, certain records matter early in the process:

• Recent pay stubs for each parent covering several months
• Complete tax returns from recent years
• Documentation of self employment income or business deductions
• Records of health insurance premiums and childcare costs paid for the child

How Shared Parenting Time Changes The Calculation

When both parents have a substantial amount of overnight parenting time, Illinois applies an adjusted version of the Income Shares formula that accounts for both households directly covering day to day expenses. This shared parenting calculation can meaningfully change the final number compared to a standard sole custody arrangement.

Getting An Accurate Calculation In Granite City

Because disputes over what counts as income, or how self employment earnings should be characterized, can shift the calculation significantly, careful documentation from both sides matters before an agreement is reached. A Granite City family lawyer can review income records from both parents to confirm the resulting number reflects an accurate picture.

Modifying A Support Order As Circumstances Change

Because the Income Shares calculation depends on current income figures, a significant change in either parent’s earnings can justify revisiting an existing order. A Granite City family lawyer can evaluate whether a modification is worth pursuing based on how much has changed since the original calculation.

Since the formula depends so heavily on documentation from both parents, disagreements over income figures often shape the outcome more than either parent initially expects. If you need help calculating or modifying child support in the Granite City area, reach out to our office to go over your specific income details.