Illinois Child Support Calculator

Illinois child support law underwent its most significant overhaul in decades when Public Act 99-764 took effect on July 1, 2017. Before that date, Illinois used a simple Percentage of Net Income model that looked only at the paying parent's income: 20% of net income for one child, 28% for two, 32% for three, 40% for four, 45% for five, and 50% for six or more children. While easy to administer, critics argued that this approach ignored the receiving parent's financial capacity and produced unfair results when the obligee earned significantly more or less than the obligor. The Legislature responded by adopting the Income Shares model used by a majority of states, which accounts for both parents' incomes in determining the support obligation and more accurately reflects the financial context of the entire family rather than just one parent.

Under the Income Shares philosophy, child support is designed to approximate what both parents would have spent on the child if they had remained living together as a household. The model starts from the premise that children should receive the same proportion of parental income they would have had in an intact family. By combining both parents' net incomes and applying a schedule that estimates child-rearing expenditures at various income levels, Illinois courts arrive at a basic obligation that reflects the realistic financial context of both households. This approach tends to produce lower obligations when the receiving parent earns a high income and higher obligations when the receiving parent has little or no income, creating a more equitable distribution of the child-rearing financial responsibility between the two households.

The transition to income shares also created a new framework for shared parenting situations. Under the old percentage model, there was no formal mechanism to reduce support when the non-custodial parent exercised substantial parenting time. The 2017 legislation introduced a shared parenting credit that reduces the basic child support obligation when the non-primary parent has 146 or more overnights per year with the child — roughly 40% of all nights annually. The credit is calculated using a formula that accounts for the number of overnights above the threshold, creating a graduated reduction rather than a cliff effect. This change was particularly significant for parents in joint parenting arrangements who previously had no formal adjustment mechanism to recognize the direct costs they were bearing during their substantial parenting time.

Illinois uses each parent's net income — not gross income — as the starting point for the calculation. Under 750 ILCS 5/505, net income means total income from all sources minus certain enumerated deductions. Allowable deductions include federal and state income taxes actually paid, Social Security and Medicare taxes, mandatory retirement contributions required by law or as a condition of employment, mandatory union dues, health insurance premiums paid for the parent personally, prior child support orders actually being paid for other children, and a self-employment tax deduction for the employer share of FICA for self-employed parents. The statute is specific about which deductions are permitted, and courts are not authorized to allow deductions that do not appear on the statutory list, which prevents creative accounting from reducing apparent income below actual financial capacity.

Once each parent's net income is calculated, the two figures are added together to produce a Combined Adjusted Net Income. Illinois maintains a schedule — formally titled the Income Shares Schedule for Basic Support Obligations — that shows the basic monthly support amount for various combined income levels and numbers of children. This schedule was built using economic data on child-rearing expenditures and is updated periodically to reflect current cost data. The table reflects the estimated percentage of household income that families at various income levels spend on child-related expenses. Finding the right cell in the table for a family's combined income and number of children produces the Basic Support Obligation, which represents the total combined contribution both parents are expected to make toward the child's financial needs.

Each parent's individual obligation is their proportional share of the Basic Support Obligation. The proportion is calculated by dividing each parent's net income by the combined net income. For example, if the non-custodial parent earns $4,000 in net monthly income and the custodial parent earns $2,000, the combined income is $6,000. The non-custodial parent's share is $4,000 divided by $6,000, or 66.7%, making them responsible for 66.7% of the Basic Support Obligation. The custodial parent's share of 33.3% is presumed to be spent directly on the child through the costs of day-to-day care including housing, food, transportation, and supervision. The non-custodial parent's proportional share is then paid to the custodial parent as the formal child support obligation in the court order.

Illinois provides a significant financial adjustment — called the Shared Physical Care Credit — when the non-primary parent has substantial parenting time. The threshold is 146 overnights per year, which works out to approximately 40% of all nights in a year. Parents with fewer than 146 overnights do not qualify for any automatic reduction in support under this provision. When a parent crosses this threshold, the adjustment is triggered and the basic obligation is recalculated using a multi-step formula that was written into the 2017 statute as part of the Income Shares reform. This threshold was chosen to reflect genuine shared physical care rather than merely a generous visitation schedule that still leaves one parent as the primary residential caregiver bearing the majority of daily costs.

The shared parenting adjustment formula has several steps. First, multiply each parent's share of the Basic Support Obligation by 1.5. This inflated figure accounts for the additional fixed costs each parent incurs by maintaining a home capable of housing the child — costs that exist regardless of how many nights the child is actually present in that home. Then, multiply each inflated obligation by the other parent's percentage of parenting time. Add the two resulting products together to produce a shared support amount. Each parent's individual obligation under the shared formula is the higher-earning parent's product minus the lower-earning parent's product, with the higher earner making the payment to the lower earner. If both parents have exactly equal income and equal overnights, the formula produces zero support in either direction.

Beyond the formal parenting time credit, Illinois courts also have discretion to deviate from the shared parenting calculation in specific circumstances. If the court finds that the standard shared parenting formula produces an amount that would be inappropriate given the specific circumstances of the children, the parents, or the available resources, it may enter a different figure accompanied by written findings explaining the deviation. Common grounds for deviation in shared parenting cases include a significant disparity in housing quality between the two homes, a child with special needs whose intensive care is concentrated primarily in one home, or one parent's income being so much higher than the other's that strict formula application would produce an unjust result for the lower-earning parent's household.

Illinois adds healthcare and childcare costs directly to the Basic Support Obligation before calculating each parent's proportional share. Health insurance premium costs attributable to the child — typically the additional premium cost of adding the child to an existing policy — are allocated between the parents in proportion to their net incomes. Illinois courts generally order the parent who has access to the most cost-effective coverage through an employer to carry health insurance on the child, with the other parent contributing their proportional share of the premium cost. When both parents have access to comparable employer coverage, the court will determine which plan offers better benefits at lower cost and order accordingly, then split the premium cost proportionally.

Work-related childcare costs — the amounts paid for daycare, after-school care, or other childcare that allows a parent to work or attend job training — are similarly added to the support calculation and split proportionally between the parents. Illinois does not limit the add-on to a particular type of childcare arrangement; any childcare costs that are reasonably necessary to allow the custodial parent to maintain employment qualify for inclusion. Courts may scrutinize unusually high childcare costs to ensure they reflect actual and necessary expenses rather than premium care choices that exceed what is reasonable for the family's income level. Changes in childcare costs — such as a child beginning school and no longer needing full-time daycare — can be a basis for modifying the support order downward.

Extraordinary medical expenses not covered by insurance are handled separately from the ongoing support order. These are typically addressed through a cost-sharing arrangement — often proportional to income — that is included in the initial support order. When extraordinary medical expenses arise, the parent who incurs them notifies the other parent and provides documentation. The other parent's share is then paid directly or deducted from future support payments, depending on the order's specific language. Illinois courts take a practical approach to these provisions, recognizing that unexpected medical costs can be significant and that both parents should bear responsibility proportional to their financial capacity. Dental and vision costs not covered by insurance are typically treated the same way as other extraordinary medical expenses and reimbursed through the same cost-sharing mechanism.

Illinois courts can deviate from the guideline amount when strict application would be inappropriate. The statute lists specific factors courts must consider: the financial resources of the child, the financial needs and resources of each parent, the standard of living the child would have enjoyed if the parents had not divorced, the physical and emotional condition of the child and their educational needs, whether either parent is financially supporting another household, the tax consequences to each party, the reasonable needs of the child, and any agreement between the parties about support. Unlike states that simply authorize deviation for good cause, Illinois provides this detailed framework to guide judicial discretion and ensure consistency across the state's circuit courts in determining when departure from the guideline is truly appropriate.

Low-income parents receive special consideration under the Illinois guidelines. If the non-custodial parent's net income is below $1,250 per month, the court is required to carefully evaluate the appropriateness of any support order and apply a self-sufficiency reserve. Illinois recognizes the parent's need to maintain a minimum standard of living while still contributing to their child's support. Courts often set nominal orders — sometimes as low as $40 to $50 per month — for parents in truly impoverished circumstances, both to preserve the ongoing obligation and to establish a modifiable record for the future if the parent's income improves. The order does not disappear because the obligor is low-income; rather, it is set at a level that can realistically be paid and enforced without driving the obligor into deeper poverty.

Modifications in Illinois require a showing of a substantial change in circumstances since the last order. Common qualifying changes include a significant increase or decrease in either parent's income, a change in the number of children covered by the order, a change in the custody or parenting time arrangement, or a significant change in the child's medical or educational needs. Illinois also allows a petition for modification if three years have passed since the last order and the current amount would differ by more than 20% from the amount a court would order today using current incomes and the current guidelines schedule. This periodic review mechanism ensures that orders do not become permanently stale when family circumstances shift significantly over time, while the 20% threshold prevents constant relitigation over minor income fluctuations.