California Child Support Guideline Formula: Income, Deductions, and Add-Ons

California child support calculation runs on a statutory formula in Family Code Section 4055 that takes each parent’s net monthly disposable income and the percentage of time the higher-earning parent has the children, then produces a dollar figure the court is presumed to enter. The formula is the same in every county. The inputs change the result far more than any argument about the formula ever will, so most of the work in a support case is proving what each parent earns and how custodial time is actually split.

The Guideline Formula

The equation is CS = K[HN − (H%)(TN)].¹ Reading the variables:

CS is the monthly child support amount.
HN is the higher-earning parent’s net monthly disposable income.
TN is both parents’ combined net monthly disposable income.
H% is the approximate share of time the higher earner has physical responsibility for the children.
K is a factor representing the portion of combined income allocated to child support.

K is where the formula bites. It equals (1 + H%) when H% is at or below 50%, or (2 − H%) when H% is above 50%, multiplied by a base fraction that depends on combined income:

$0–$800 combined monthly income: 0.20 + TN/16,000
$801–$6,666: 0.25
$6,667–$10,000: 0.10 + 1,000/TN
Over $10,000: 0.12 + 800/TN

These brackets are fixed in the statute and have not changed since the formula was enacted.¹

When parents have different time-sharing arrangements for different children, H% is averaged across all of them. One child living primarily with the mother and another splitting time equally gets blended into a single H% for the calculation, which can meaningfully change the output compared with running each child separately.

Almost nobody computes this by hand. California courts use certified software, and the Judicial Council has recertified Xspouse, Family Law Software, and FamilySoft SupportCalc for 2025–26. In cases handled by the local child support agency, the court is required to use the state’s own California Child Support Guideline Calculator.² The reliable way to see what you will owe or receive is to run those numbers through one of the certified calculators with honest inputs.

What Counts as Income

California defines annual gross income broadly as income from whatever source derived. That includes wages, salaries, commissions, bonuses, royalties, rental income, dividends, interest, trust income, pensions, Social Security benefits, workers’ compensation, unemployment and disability benefits, severance pay, certain veterans’ benefits, and military housing and food allowances. Spousal support received from someone who is not a party in the current case counts too.³

Self-employment income is gross business receipts minus the expenses required to run the business. The court can also count employee benefits like a company car or housing allowance when those reduce the parent’s actual living costs. Child support received for children from a different relationship is not income, and neither is need-based public assistance.³

When the Court Uses Earning Capacity Instead

If a parent’s income is unknown, the court must consider earning capacity. If income is known but the parent is working below their potential, the court has discretion to use earning capacity instead of actual earnings, as long as doing so serves the children’s best interests. Judges look at work history, job skills, education, health, age, criminal record, and the local job market. A parent who quits a high-paying job or takes a lower-paying position may have support calculated as if they were still earning at the prior level. One exception: a parent who is incarcerated or involuntarily institutionalized cannot be treated as voluntarily unemployed, regardless of the offense.³

Deductions That Produce Net Disposable Income

The formula runs on net, not gross. Family Code Section 4059 lists the deductions:

State and federal income taxes actually owed given filing status and credits, not just current withholding
FICA (the employee share, or an equivalent amount for self-employed workers)
Mandatory union dues and retirement contributions required as a condition of employment
Health insurance premiums for the parent and for any children the parent must support, plus state disability insurance premiums
Child or spousal support actually being paid under a court order for people not involved in the current case
Job-related expenses at the court’s discretion, considering whether the expenses are necessary
Hardship deductions for extraordinary health expenses, catastrophic losses, or the basic living expenses of other children in the parent’s home

The hardship deduction for other children living with you cannot exceed the per-child amount being set in the current order.⁴⁵

A deduction only counts if you can document it. Claiming union dues without a pay stub or employer letter is the fastest way to lose that deduction. Watch the tax line in particular: overwithholding (the kind that produces a refund every spring) inflates the deduction and lowers the resulting support figure, and the court is supposed to use what you actually owe.

Add-On Expenses Beyond the Base Amount

The formula covers general child-rearing costs. Several specific expenses sit outside it, under Family Code Section 4062.

The court must add:

Childcare tied to work or job training
Uninsured medical, dental, and vision costs for the children

The court may add, at its discretion:

Educational or special needs costs such as tutoring, private school tuition, or therapy
Travel for visitation when a parent lives far away

Add-ons are generally divided between the parents in proportion to their net disposable incomes, not split evenly. A parent earning 70% of the combined income typically covers 70% of these costs.⁶ This means the number the calculator spits out is rarely the full picture of what the paying parent actually transfers each month.

The Low-Income Adjustment

California builds in a safety valve for low earners. If the paying parent’s net monthly disposable income is less than the gross monthly income from full-time minimum wage work (40 hours per week, 52 weeks per year), that parent is presumptively entitled to an adjustment that reduces the support amount.⁷

The reduction scales. The numerator of the adjustment fraction is the full-time minimum wage gross income minus the parent’s actual net disposable income; the denominator is the full-time minimum wage gross income. A parent just below the threshold gets a small adjustment, a parent far below gets a larger one. Certified calculator software applies this automatically. The other parent can rebut the presumption by showing the adjustment would be unjust.⁷

When Judges Depart from the Formula

The guideline amount is presumed correct, and the presumption can be overcome only on specific grounds. Family Code Section 4057 requires the judge to state on the record why the formula figure does not serve the child’s best interests before entering a different number.⁸ Common grounds:

Both parents stipulate to a different amount and the court finds it adequate
The paying parent’s income is extraordinarily high and the formula amount exceeds what the children actually need
A parent with custodial time is not spending at a level consistent with that time
Even after the low-income adjustment, the amount exceeds 50% of the paying parent’s net disposable income
Deferred sale of the family home where the rental value exceeds mortgage, insurance, and property taxes combined
Special circumstances such as significant medical needs, more than two legal parents, or substantially equal custody with very different housing cost burdens

Deviations are the exception. The party asking for one carries the burden of proof, and judges are generally reluctant to stray from the formula without clear documentation that the result would otherwise be unfair.⁸

Running Your Own Numbers

Both parents must file an Income and Expense Declaration (Form FL-150) disclosing earnings, assets, and monthly expenses.⁹ Parents with straightforward finances may use the simplified FL-155. Either form requires recent pay stubs, your most recent tax return, and documentation of any other income.

Accuracy is not optional. Courts can sanction parents for misrepresenting income or hiding assets, and the penalties escalate when the misrepresentation is intentional. Beyond sanctions, incomplete disclosure usually backfires: a judge who suspects underreporting may impute a higher earning capacity based on lifestyle, spending patterns, or prior tax returns.

To get a realistic estimate before court, gather the same documents you will file: pay stubs for both parents if you have them, your most recent tax return, proof of health insurance premiums, proof of any other support orders you pay, and an honest count of overnights per year with each parent. Enter them in one of the certified calculators listed above. The number that comes out is what the court will start with.

Changing the Amount Later

Support orders are not permanent. Under Family Code Section 3651, a court can modify or terminate an order when circumstances make it necessary.¹⁰ Common triggers include a significant change in either parent’s income, a shift in custody, a child’s new medical needs, or a job loss. Quitting voluntarily to reduce the obligation invites the earning capacity analysis described above.

One timing rule governs everything: a modification cannot be made retroactive to any date before the motion is filed. Every payment that came due before your filing date is locked in, no matter how drastically your circumstances changed. Lose your job on January 1 but wait until March 15 to file, and you owe the full original amount for January and February. The new amount can backdate to March 15 at the earliest. File the day the change happens, not when you get around to it. Military service members activated and deployed out of state can use a simplified notice of activation instead of the standard motion.¹⁰

1
California Legislative Information. California Family Code Section 4055
2
Judicial Branch of California. Guideline Support Calculators
3
California Legislative Information. California Family Code Section 4058
4
California Legislative Information. California Family Code Section 4059
5
California Legislative Information. California Family Code Section 4071
6
California Legislative Information. California Family Code Section 4062
7
California Legislative Information. California Family Code Section 4055
8
California Legislative Information. California Family Code Section 4057
9
California Courts Self-Help Guide. Income and Expense Declaration
10
California Legislative Information. California Family Code Section 3651