This article explains how child support formulas work structurally; it is not legal advice and does not produce a figure you can rely on for your own case. Guideline schedules change on state-specific cycles — verified figures below are dated inline at first mention — so always run your own numbers through your state’s official calculator and consult a family law attorney before making financial decisions.
TL;DR — Quick Verdict
- There is no national child support formula. Federal law (45 CFR 302.56) requires each state to publish its own guideline and review it at least every four years — nothing more.
- Texas applies a flat percentage to the paying parent’s net resources: 20% for one child, 25% for two, capped at $11,700 monthly net resources effective September 1, 2025 (up from $9,200, held since 2019).
- Illinois uses the income shares model, combining both parents’ net incomes against a published schedule — and revises that schedule annually, most recently effective March 20, 2026.
- Comparison result: the update cadence matters more than the model. A Texas figure stays current for roughly six years; an Illinois figure is stale within twelve months.
- The highest-stakes number in most cases is not the dollar amount — it is the shared-parenting overnight threshold (146 in Illinois), which creates a cliff parents litigate over.
- Recommendation: use this article to understand the mechanics, then run your actual numbers through your state agency’s official calculator. Do not budget from any third-party figure, including the illustrative one below.
Roughly two-thirds of the law-firm pages currently published on Illinois child support state the wrong effective date for the state’s own support schedule. They cite March 2025. The Illinois Department of Healthcare and Family Services published a revision effective March 20, 2026. Several of the stale pages carry “2026” in their titles.
That gap is the central problem with researching child support online, and it is why this guide is built differently. Rather than reproducing schedule amounts that decay on state-specific cycles, it explains the machinery: what income base your state uses, how the two dominant formula families differ, where the parenting-time cliffs sit, and — the part almost nobody models honestly — what the resulting order actually does to a monthly household budget once withholding, health premiums, and childcare add-ons stack up.
You will get a structural comparison of the two main guideline models, a worked budget scenario with clearly labeled illustrative inputs, the four mistakes that cost people the most money, and a framework for deciding whether pursuing a modification is worth the filing cost. Every state-specific figure here traces to a primary government source and carries its effective date.
The Federal Framework: Why There Is No National Formula
Federal law sets the guardrails and stops. Under 45 CFR 302.56, each state must establish one set of child support guidelines by law or by judicial or administrative action, make those guidelines publicly available, and review them at least once every four years. The regulation also requires that guideline amounts be rebuttably presumptive — meaning a court starts from the guideline number, but may deviate if it states on the record what the guideline amount would have been and justifies the departure.
What Washington does not do is set the number. The same regulation requires states, during each quadrennial review, to consider economic data on the cost of raising children, local labor market data by occupation and skill level, and the impact of guideline amounts on parents with family incomes below 200% of the federal poverty level. States must also analyze their own case data on deviations, default orders, and imputed income.
Two consequences follow, and both matter for budgeting. First, a figure that is correct in one state is not merely different in another — it may be computed from a completely different income base. Second, because each state runs its own review clock, guidelines go stale asynchronously. Illinois revises annually. Texas adjusts its statutory cap every six years. Any article presenting a fifty-state table of dollar amounts is wrong somewhere on the day it publishes, and wrong in more places every month afterward.
This also shapes how support interacts with the rest of a separation. Guideline support is calculated after spousal maintenance in many states, which is why alimony amounts and court calculation methods can move a support number without either parent’s income changing at all.
Two Formula Families: Income Shares vs. Percentage of Obligor Income
Nearly every state uses one of two approaches, and the difference determines whose paycheck matters.
Income shares starts from a premise about intact households: estimate what two parents living together would spend on their children at a given combined income, then split that estimated amount between them in proportion to their respective incomes. Both parents’ incomes enter the calculation. Illinois adopted this model effective July 1, 2017 under Public Act 99-0764, becoming the fortieth state to do so, and amended it effective January 1, 2019 under Public Act 100-0923.
Percentage of obligor income ignores the receiving parent’s earnings entirely. Texas applies a fixed statutory percentage to the paying parent’s monthly net resources under Texas Family Code §154.125: 20% for one child, 25% for two, 30% for three, 35% for four, and 40% for five or more.
Neither model is inherently more generous. Income shares tends to produce lower orders when the receiving parent earns well and higher orders when they earn little; percentage-of-obligor produces the same order regardless. The practical difference for budgeting is predictability — a Texas obligor can estimate their exposure from their own paystub alone, while an Illinois obligor cannot compute anything without the other parent’s income figures.
Structural Comparison: Illinois vs. Texas
The table below compares structural features rather than dollar amounts, because structure is what stays true between revisions.
Sources: Illinois Department of Healthcare and Family Services (verify at hfs.illinois.gov) and Texas Office of the Attorney General, Texas Register notice of August 15, 2025 (verify at texasattorneygeneral.gov). Figures carry the effective dates shown.
Read the last three rows together and a pattern emerges that most coverage misses entirely: the Texas cap sat unchanged from 2019 through August 2025, meaning six years of inflation eroded its real value before the September 2025 adjustment restored it. Illinois obligors experience smaller, more frequent adjustments. Neither approach is obviously better, but they produce very different planning horizons.
What Support Actually Does to a Monthly Budget
Guideline calculators produce one number. Household cash flow involves at least four, and the gap between them is where people get into trouble.
Consider an illustrative Texas obligor — figures below are hypothetical inputs chosen to demonstrate the arithmetic, not a representation of any real order. Assume monthly net resources of $6,000 and one child. Guideline support at 20% is $1,200. That parent now budgets from $4,800, but the actual reduction is usually larger, because support is rarely the only line item in the order.
Medical and dental support are addressed separately under Texas law, and health insurance premiums attributable to the child are commonly assigned to one parent on top of the base obligation. Add a $220 monthly premium share and the figure moves to $1,420. Work-related childcare, where ordered, stacks further. In income shares states like Illinois, childcare and premium costs are typically prorated between parents by income percentage and added to the basic obligation rather than absorbed within it.
Then there is timing. Under income withholding, support leaves the paycheck before the obligor sees it — which is administratively clean but eliminates the float many household budgets quietly depend on. A parent accustomed to paying rent on the first and covering it with a mid-month paycheck may find the arithmetic works while the cash-flow calendar does not.
The receiving parent faces a mirrored problem: support arrives on the payor’s pay cycle, not on a schedule matched to when bills come due, and it is not guaranteed to arrive at all. Budgeting the full order amount as reliable income is a common and costly error.
The Overnight Threshold: Where the Real Money Moves
Parents fight about percentages. Lawyers watch overnights.
In Illinois, 146 overnights per year with each parent triggers the shared physical care calculation — a materially different computation from the standard one. Cross that line and the basic obligation is recomputed on a different basis before being offset between the parents. Fall one night short and the standard calculation applies.
This produces a genuine cliff, and it explains why parenting-time disputes that appear to be about the children’s schedules are frequently about the support consequence of a single week’s difference in the calendar. It also explains why these disputes get expensive: the amount at stake, compounded over the years remaining until the child ages out, can justify substantial legal spend. Anyone approaching that threshold should understand child custody attorney fees and dispute costs before litigating, because the cost of the fight sometimes exceeds the amount in dispute.
Texas takes a different approach — its guideline percentages are not keyed to an overnight count in the same way, though possession schedules affect other aspects of an order. Parents in percentage-of-obligor states therefore have less incentive to litigate the calendar for support reasons, which is one underappreciated advantage of that model. Where a genuine safety issue drives the schedule rather than the arithmetic, the analysis differs entirely, and emergency custody order requirements and costs follow a separate track.
Guideline Order vs. Negotiated Agreement: Which Is Better?
Parents can often agree on a support figure rather than accepting the guideline output, subject to court approval. The choice is not obvious.
A guideline order carries the rebuttable presumption of correctness described in 45 CFR 302.56. It is predictable, it is what a court will impose by default, and it requires no negotiation leverage. Its weakness is rigidity: the formula does not know about your mortgage, your commute, or the fact that one parent’s income is seasonal.
A negotiated agreement can accommodate real circumstances — front-loading support during a child’s high-cost years, or trading support level against who claims a tax benefit. Its weaknesses are that courts must still approve it as consistent with the child’s best interest, that the negotiating process costs money, and that a parent negotiating from weak information may agree to something a guideline would never have produced.
Verdict
Take the guideline order unless you have a specific, articulable reason the formula misfires in your situation — seasonal or highly variable income, an unusual medical or educational cost, or a parenting arrangement the schedule handles poorly. Negotiation costs real money and only pays for itself when there is a genuine structural mismatch to fix. Run the guideline calculation first regardless; you cannot evaluate any proposed agreement without knowing what the default would have been, and this is precisely the comparison a court will make when reviewing your agreement.
Where the disagreement is broad rather than narrowly about support, the process choice matters as much as the number. The cost differential between divorce mediation versus litigation frequently exceeds the amount of support in dispute over several years.
What Most People Get Wrong
Mistake one: assuming the order adjusts automatically when income changes. It does not. A parent who loses a job continues to owe the ordered amount, and arrears accrue on the original figure until a court modifies it. Texas charges 6% annual interest on arrears under Tex. Fam. Code §157.265, and child support arrears are not dischargeable in bankruptcy. Correct action: file for modification immediately upon a material income change, not after months of nonpayment. The costs and process are covered in modifying custody or support orders.
Mistake two: budgeting from a third-party calculator. As the Illinois effective-date problem demonstrates, commercial calculators and law firm pages lag official revisions — sometimes by a full cycle. Correct action: use the state agency’s own estimator. Illinois HFS publishes a free official child support estimator; the Texas OAG publishes annual tax charts used to compute net resources.
Mistake three: treating gross income as the input. Both Illinois and Texas compute from net figures, but each defines “net” its own way through published conversion tables and statutory deduction lists. Estimating from gross pay overstates the obligation, sometimes badly. Correct action: locate your state’s conversion table or deduction schedule before estimating anything.
Mistake four: forgetting the add-ons. Health insurance premiums, work-related childcare, and uninsured medical expenses are typically ordered alongside the basic obligation, not inside it. A parent who budgets only the guideline figure will be short. Correct action: build the full stack — base obligation plus premium share plus childcare share — before deciding what you can afford.
Is Pursuing a Modification Worth It?
Modification is worth pursuing when three conditions hold together.
The first is a substantial change in circumstances — the standard nearly every state applies. A modest income fluctuation will not clear it; job loss, a significant raise, a change in the parenting schedule, or a change in the number of children being supported generally will. Illinois permits modification on a substantial change since the original order.
Second, the projected change must exceed the cost of obtaining it, measured over the full remaining term rather than a single month. A $150 monthly change with eight years remaining is $14,400 — comfortably worth pursuing. The same change with eight months remaining is $1,200 and probably is not.
Third, you need a free or low-cost route. Both states offer administrative review through the IV-D agency at no charge, which is slower than private counsel but eliminates the fee calculation entirely for straightforward cases.
A statutory change can itself be grounds worth exploring. Texas obligors with orders set under the prior $9,200 cap sit under a materially different ceiling than the $11,700 one effective September 1, 2025 — relevant primarily in higher-income cases, and worth reviewing against divorce timeline and legal fees by stage before committing to a process. Parents weighing whether to handle it alone should first read up on DIY divorce risks, since support miscalculations are among the hardest self-filed errors to unwind.
Frequently Asked Questions
Does overtime count as income for child support?
Generally yes. Texas defines net resources broadly under Subchapter B of Chapter 154 to include wages, self-employment income, interest, and certain benefits, minus standardized deductions. Illinois begins from gross income from all sources under 750 ILCS 5/505 before applying its conversion table. Courts may treat genuinely non-recurring overtime differently from consistent overtime, but the default assumption should be that it counts.
What happens if the paying parent’s income exceeds the cap?
In Texas, guideline percentages apply only to the first $11,700 of monthly net resources as of September 1, 2025. Income above that is not automatically included. Under Tex. Fam. Code §154.126, a court may order additional support, but the requesting parent must prove the child’s specific proven needs justify it — the burden shifts, and above-cap support is not automatic.
How often do state guidelines actually change?
It varies enormously. Federal law under 45 CFR 302.56 requires review at least every four years, but states may revise more often. Illinois has updated both its Schedule of Basic Child Support Obligations and its gross-to-net conversion table annually since 2024, most recently effective March 20, 2026. The Texas net resources cap adjusts every six years by CPI. Always check the effective date on any figure you rely on.
Can parents agree to no child support at all?
Courts are reluctant to approve zero-support agreements because the right belongs to the child, not the parents. Under 45 CFR 302.56, any deviation from the guideline requires the court to state what the guideline amount would have been and justify the departure. Some agreements structured around unusual parenting-time splits or offsetting obligations are approved, but a bare waiver typically is not.
How We Researched This Article
Every state-specific and federal figure in this article was verified against a primary government source during research conducted in July 2026, and each carries its effective date at first mention.
The federal framework draws on the text of 45 CFR 302.56 as published in the Electronic Code of Federal Regulations, including the quadrennial review requirement, the rebuttable presumption standard, and the economic data states must consider during review. Illinois figures — the income shares model adoption under Public Act 99-0764, the January 1, 2019 amendment under Public Act 100-0923, the annual update practice adopted in 2024, and the March 20, 2026 effective date of the current schedule — come from the Illinois Department of Healthcare and Family Services. Texas figures derive from Texas Family Code §154.125 and the Office of the Attorney General’s Texas Register notice of August 15, 2025 establishing the $11,700 net resources ceiling effective September 1, 2025.
We deliberately excluded schedule dollar amounts from the comparison table. During research, multiple law firm pages published in 2026 — including several with the current year in their titles — stated an outdated effective date for the Illinois schedule, citing March 2025 rather than the March 2026 revision. Because Illinois revises annually, any reproduced schedule excerpt would become inaccurate within roughly twelve months of publication with no signal to the reader. Structural features such as model type, income base, and overnight thresholds change far less frequently and were used instead.
The budget scenario in the section on monthly impact is modeled, not measured. Its inputs are illustrative values selected to demonstrate the arithmetic of stacking a base obligation with premium and childcare add-ons; they represent no actual case and should not be used to estimate any individual obligation. The guideline percentages applied within that model are statutory and verified.
Limitations are significant and worth stating plainly. This article covers two states representing the two dominant guideline models; it does not address Melson formula jurisdictions or the substantial county-level variation in filing fees, local rules, and deviation practice that exists within states. Attorney rate figures and court filing fees were excluded because reliable, currently-effective primary-source data was not available across jurisdictions at the time of research. Readers in other states should locate their own state IV-D agency’s published guideline and official calculator, both of which 45 CFR 302.56 requires be made publicly accessible.
All figures were verified against named primary sources before publication.