California calculates child support using a mandatory statewide formula, not a judge's discretion. This guide explains how the guideline works, what goes into it, the add-on costs, how long support lasts, and how it is modified and enforced.
This is general information about California law, not legal advice. Consult a California-licensed family law attorney about your situation.
The statewide guideline formula
California sets child support by a uniform guideline formula in Fam. Code § 4055. The formula combines both parents' net disposable incomes and the percentage of time each parent has primary physical responsibility for the child (the "timeshare"). In broad terms, the more a parent earns relative to the other and the less time that parent spends with the child, the higher their support obligation. The guideline amount is presumptively correct, and courts use a certified calculator (commonly the DissoMaster or a Judicial Council–approved calculator) to apply the formula. Judges may depart from the guideline only in limited, stated circumstances.
The statutory formula is written as CS = K (HN − (H% × TN)). In that equation, CS is the support amount, K is a fraction representing the combined total of both parents' income allocated to child support, HN is the higher earner's net monthly disposable income, H% is the higher earner's share of parenting time, and TN is the combined net monthly disposable income of both parents. No one is expected to compute this by hand — the certified calculator does it — but the structure explains two things people find counterintuitive: support rises as the income gap widens, and it falls as the paying parent's timeshare increases. The two biggest levers in any case are therefore the parents' net incomes and the percentage of time the child spends with each.
What counts as income
Income for support is defined broadly (Fam. Code § 4058): wages, salary, bonuses, commissions, self-employment income, rental income, dividends, interest, investment returns, pensions, unemployment and disability benefits, workers' compensation, and Social Security benefits, among others. From gross income the formula subtracts certain items — taxes, mandatory union dues and retirement contributions, health-insurance premiums, and other child or spousal support actually paid — to reach net disposable income. Courts can also impute income to a parent who is deliberately unemployed or underemployed, based on their earning capacity and opportunity to work. Because accurate figures are central, the mandatory Income and Expense Declaration (Form FL-150) is the backbone of every support case.
The 2024 guideline update (SB 343)
California overhauled its child support rules with Senate Bill 343, which took effect September 1, 2024 — the most significant revision to the guideline in roughly three decades. The changes did not throw out the basic formula, but they refined several inputs and strengthened protections for lower-income parents. The law codified the low-income adjustment and tied its threshold to what a parent would earn working full time at the current state minimum wage (40 hours per week, 52 weeks per year), so the cutoff rises automatically as the minimum wage rises. It also reinforced a self-support reserve — a floor meant to ensure a paying parent retains enough income to meet basic needs — and clarified how add-on expenses like childcare and uninsured medical costs are allocated. The practical takeaway is that orders entered before September 2024 may produce a different result if recalculated today, which can be a basis to revisit an older order.
Mandatory and discretionary add-ons
On top of the base guideline amount, California requires courts to add certain costs (Fam. Code § 4062), typically split between the parents in proportion to their incomes: childcare needed so a parent can work or get job training, and the child's reasonable uninsured health-care costs. Courts may also order discretionary add-ons such as the child's educational or other special needs and travel expenses for visitation. These add-ons are separate from the base support number and are usually shared one-half each unless the court orders a different split.
A worked example
Imagine Parent A nets $7,000 a month and Parent B nets $3,000, and the child lives with Parent B about 70% of the time. Because Parent A earns more and has the smaller timeshare, the formula assigns Parent A the support obligation; the calculator combines the two net incomes, applies the K factor, and subtracts the value attributable to Parent A's parenting time. Suppose the certified calculator returns a base figure of roughly $1,100 per month. On top of that, the parents pay add-ons in proportion to income: with Parent A earning 70% of the combined net income, Parent A would cover about 70% of work-related childcare and the child's uninsured medical costs, and Parent B about 30%. If the timeshare later shifts — say Parent A's parenting time climbs from 30% to 45% — the base support figure drops, because the formula credits the additional time the paying parent directly supports the child. This is why disputes over the parenting schedule and disputes over support are so tightly linked.
Deviating from the guideline
Because the guideline figure is presumptively correct, a court can order a different amount only on specific grounds — for example, when a parent has an extraordinarily high income such that the guideline amount would exceed the child's needs, when a parent isn't contributing to the child's needs in proportion to their timeshare, or when applying the formula would be unjust or inappropriate in a particular case. Parents can also agree to a different amount (called "stipulated" support), but only if the court is satisfied the amount adequately meets the child's needs and the parents understand their guideline rights.
How long support lasts
Child support in California generally continues until the child turns 18, or until 19 if the child is still a full-time high school student living at home (Fam. Code § 3901), whichever comes first. Support for an adult child can continue where the child is incapacitated from earning a living and without sufficient means. Parents are free to agree to support a child through college, and a court can enforce that agreement, but a court generally cannot order college support absent the parents' agreement.
Step by step: establishing child support
- Open or use a case — support is set within a divorce, legal separation, parentage case, or a case opened by the local child support agency.
- Exchange income information by completing the Income and Expense Declaration (Form FL-150) with proof of earnings.
- Run the guideline using a certified calculator with both parents' net incomes and the timeshare.
- Add the mandatory add-ons for childcare and uninsured health costs (Fam. Code § 4062).
- Obtain the order by stipulation or after a hearing on a Request for Order (Form FL-300).
- Modify later by filing a new request whenever there is a material change in circumstances.
Modifying support
A support order can be modified when there is a change in circumstances — such as a significant change in either parent's income, a job loss, a new child, or a change in the parenting timeshare. Either parent can ask the court to recalculate the guideline. Importantly, a modification generally takes effect no earlier than the date the request is filed, so a parent whose income drops should file promptly rather than wait; support already accrued under the existing order is not retroactively erased.
A few changes are common enough to flag. A job loss or significant pay cut is a classic basis to lower support — but only from the filing date forward, which is why prompt filing matters. A raise or new high-paying job for the paying parent can justify an increase. A shift in the parenting timeshare changes the formula directly, so a parent who begins spending substantially more time with the child has grounds to recalculate. And because SB 343 changed several inputs as of September 2024, an order calculated under the old rules may itself warrant a fresh look. What is not a self-help option is simply paying less than the order says: only the court can change the amount, and unpaid support keeps accruing until it does.
Enforcing support
California's Department of Child Support Services (DCSS) and its local child support agencies help establish, collect, and enforce support at no charge. Enforcement tools are powerful: wage garnishment through an earnings assignment, interception of tax refunds, suspension of driver's and professional licenses, passport denial, bank levies, and liens on property. Unpaid support (arrears) accrues interest at 10% per year and generally cannot be discharged in bankruptcy. Because arrears can pile up quickly, a parent who cannot pay should seek a modification rather than simply stop paying.
Two points compound the risk of letting arrears build. First, child support arrears generally cannot be retroactively reduced or forgiven by the court — even a judge who is sympathetic usually cannot wipe out support that already came due under a valid order. Second, the 10% annual interest is statutory and accrues automatically, so a balance that goes unaddressed grows steadily on its own. Together these rules mean that the worst response to an inability to pay is silence; the right response is a prompt modification request, which at least stops new amounts from accruing under the old, unaffordable number.
California statutes behind child support
The core authorities are all in the Family Code: § 4055 (the statewide guideline formula), § 4058 (definition of income), § 4059 (deductions to reach net disposable income), § 4062 (mandatory and discretionary add-ons), and § 3901 (duration of the support obligation). Senate Bill 343, effective September 1, 2024, updated the guideline, codified the low-income adjustment, and reinforced the self-support reserve. The guideline is meant to be applied uniformly statewide so that children in similar circumstances receive similar support regardless of which county hears the case.
Frequently asked questions
How is child support calculated in California?
By a mandatory statewide guideline formula (Fam. Code § 4055) using both parents' net incomes and the timeshare, plus add-ons for childcare and health costs. Courts apply it with a certified calculator.
Can the amount be different from the guideline?
Rarely. The guideline figure is presumptively correct; judges may deviate only in limited, stated circumstances, or where the parents agree to an amount that adequately meets the child's needs.
What income is counted?
Almost all of it (Fam. Code § 4058) — wages, self-employment, bonuses, commissions, rental and investment income, and many benefits. Courts can impute income to a parent who is deliberately unemployed or underemployed.
How long do I have to pay child support?
Generally until the child turns 18, or 19 if still a full-time high school student living at home (Fam. Code § 3901). Longer for an incapacitated adult child.
What happens if a parent doesn't pay?
Enforcement tools include wage garnishment, tax-refund interception, license suspension, and liens, often through DCSS. Arrears accrue 10% interest and are not dischargeable in bankruptcy.
What if my income changes?
Ask the court to modify support as soon as the change happens. A modification usually dates only to when the request is filed, so filing promptly protects you — stopping payment on your own does not.
Did the rules change recently?
Yes. Senate Bill 343 took effect September 1, 2024 — the biggest guideline update in about thirty years. It refined the formula's inputs, codified the low-income adjustment (with a threshold tied to full-time minimum-wage earnings), and reinforced a self-support reserve for lower-income paying parents. An order set before that date may calculate differently now.
Does how much time I spend with my child affect support?
Directly. Timeshare is one of the two main inputs to the formula. As the paying parent's percentage of parenting time goes up, the base support figure goes down, because the formula credits the time that parent is directly supporting the child. This is why custody and support disputes are so closely connected.
When to talk to a California family law attorney
Guideline child support looks mechanical, but the inputs are where cases are won or lost. Consider a California family law attorney when a parent is self-employed or has fluctuating, cash, or hidden income; when one parent may be deliberately underemployed and you want income imputed; when the timeshare is disputed (it directly changes the number); when you need to modify an existing order after a job loss or raise; or when arrears and enforcement are at issue. An attorney can make sure the certified calculator is run with the correct figures — small input errors translate into large, ongoing dollar differences.
Talk to a California family law attorney
For the bigger picture, see our complete divorce and family law guide and the child custody guide. To find a California-licensed attorney, browse the directory by practice area and county — free, no obligation.