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How the Amount of Child Support Is Determined

Although in some cases the issue of child custody would need to be dealt with first, usually the issue of child support is addressed early on in a matter. Typically, one parent will be told by the court to pay child support to the other. However, when parties share nearly equally custody, then the calculations can get complex quickly.

Child support is based on the parent’s share of income and what it costs to raise a child.

In Illinois, as in many other states, the amount of child support one parent has to pay is based on how much of the total income they are bringing in. It works like this:

  • First, the formula takes both parents’ gross income to obtain a standardized income which is then combined to make an “Adjusted Net Income” total.
  • With that total, the formula calculates each parent’s percentage share of the adjusted net income.
  • Then it looks at a chart that the state legislature made. This chart shows how much the state thinks it would cost to raise however many children there are of the relationship based on how much the parents’ total income is. This chart looks very similar to the IRS tax tables.
  • Then, based on that number, the formula will allocate the non-custodial parent’s share of that support – typically proportioned based on the percent of the total income.

For example, if one parent’s income accounts for 70% of the total, they’d be responsible for 70% of what the state thinks it would cost to rear a child.

Any money that you receive counts as income.

The statute includes more than just the money you earn from your job for purposes of calculating support. Any money that you receive is considered income. In some cases, I’ve shown that the mother received $60 a week (that she said was from a colleague “paying her back for cigarettes”) was income. This also includes money received from alimony, bonuses, legal settlements, and more.

Other variables can change the calculations.

In addition to sources of income, there are other factors that may increase or decrease the amount of support. One factor that is often overlooked is whether a parent is receiving or paying child support for another relationship. The formula also accounts for health insurance, extra-curricular activity, education, and childcare costs.

Lawyers use digital tools to make these calculations.

In practice, these calculations are generally done electronically. Your attorney may have a software program that the law firm bought, or they may use the online Illinois Child Support Estimator, which is maintained by the state Department of Healthcare and Family Services. The results may vary a little depending on which program is used, but typically the numbers aren’t going to be that different. Keep in mind that these are estimates, not final figures. You are trying to get a ballpark figure within 5% or 10% of what will be awarded by the court.

Often, I will schedule a virtual meeting with my clients, share the screen, and run through some examples together. This lets us see where we end up.

The estimators have some required fields.

First is the number of children and the number of overnights with custodial parents. So, let’s say there are two children with Mom, the custodial parent, for 300 nights a year.

Next is the gross income (before taxes, or anything else, is taken out) of each parent. So let’s say Mom makes $3,000 a month, Dad makes $5,000.

Is Mom getting any maintenance or even child support from other relationships? All sources of income need be considered.

Then you have to select whether health insurance is provided. The calculation is supposed to take the difference between the single coverage and the family plan coverage. Let’s say that single coverage is $150 and the family plan is $400, then the difference would be $250. So, that’s what it costs to provide health insurance for the two children. It’s not the $400 because they would be paying that $150 for themselves no matter what.

Example: Based on those numbers – two kids with Mom 300 nights a year, she makes $3,000 a month, Dad makes $5,000 monthly and pays $250 a month for health insurance for the kids – the estimator tells us that Dad is supposed to be paying Mom $968.38 a month in child support.

Plugging other variables into the estimator can change the picture.

You also input large expenses involving the children. Child care is usually the big one, and it can really affect the outcome, as well as affecting life decisions, such as whether the custodial parent will be able to work.

Example: So, let’s just say that Mom is earning nothing, she is not working, and you – the dad – are. With Mom staying home with the kids, you would be expected to pay $1,326 a month in child support – a significant increase from our first scenario.

Example: But if Mom is working at least part-time, that likely means she needs to make child care arrangements. Let’s say she’s making $2,500 a month, and child care is $800 a month. So her net income has increased, because now she has $1,700 after paying for child care. But what does that do to your support obligation? Let’s see in the estimator. It does drop $300 (to $1,014.24). But here is the wrinkle, your support obligation ends up being more than if she wasn’t working. Now you must pay $520.72 of the “shared child care expenses.” That gives you a total of $1,534.96.

In this example then, is it worth fighting to get mom to work part-time in that case? I’d say no. That’s why I have these kinds of “what-if scenario” conversations with my clients.

If your income goes up, your support may not drop as much as you expect.

Perhaps Mom has an opportunity to earn more than $3,000; let’s say she could make $5,000. What happens then? It can be strange how she earns $2,000 more, but it only drops Dad’s child support obligation down by about $200 (to $851.92). People ask, how is that even possible? It has to do with the formula that looks at both parents’ combined income and the cost of raising your kids.

With the increase in income, the formula probably bumped them up into a new tier where it’s expected that it’s going to cost a lot more to raise two children if you have a combined income of $10,000 a month than $8,000 a month, and therefore, the child support amount doesn’t go down as much as you thought it would.

Ultimately, the estimate is just an estimate, and the court will make the final decision. It is possible that the judge might take into consideration some other, higher extracurricular costs, or other factors that the support calculator doesn’t take into account, but in practice that rarely happens.


To sum up, I tell clients that you can’t argue with the child support figure that the estimator spits out after you put the numbers in, but you sure can fight and negotiate about what numbers get put in there. As these examples illustrate, the assumptions you make about the finances can lead to very different outcomes.


If you have questions about divorce, child custody, or child support, we can help.
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