California calculates child support using a statewide guideline formula written into Family Code § 4055. The formula is driven mainly by two things: each parent’s net disposable income and the percentage of time the child spends with each parent. Courts apply the guideline figure in nearly every case.
Unlike spousal support, child support in California isn’t left to a judge’s broad discretion — it runs through a formula that’s meant to produce a consistent, predictable number. Understanding what drives that number is the key to understanding your case.
The statewide guideline formula
California law (Family Code § 4055) sets a mathematical guideline formula that courts apply through specialized software. The guideline result is presumed to be correct, and judges depart from it only in limited circumstances. The main inputs are each parent’s income, the parenting timeshare, the number of children, and certain deductions.
The two biggest inputs: income and timeshare
Two factors do most of the work. The first is each parent’s net disposable income — gross income minus taxes and certain allowable deductions. The second is the timeshare: the approximate percentage of time the child spends with each parent. Because the formula accounts for how much time the higher earner spends with the child, custody and support are tightly linked — a change in the parenting schedule can change the support number.
What counts as income?
Income is defined broadly: wages, salary, commissions, bonuses, self-employment earnings, rental income, and more. Self-employment and irregular or cash income are frequent sources of dispute, because establishing a parent’s true earnings can take real work.
Add-ons beyond the base amount
On top of the guideline figure, certain costs are added and typically shared between the parents:
- Mandatory add-ons: child-care costs related to work or education, and the child’s reasonable uninsured health-care costs.
- Discretionary add-ons: things like educational or special needs, which a court may order in its discretion.
Can parents agree to a different number?
Yes, within limits. Parents can agree to an amount at or above guideline freely; agreeing to less than guideline requires specific findings that the child’s needs are still met. Any agreement still needs the court’s approval, because child support is considered the child’s right, not the parents’ to bargain away.
Can it change later?
Child support is always modifiable when there’s a material change in circumstances — a shift in either parent’s income or in the timeshare being the most common triggers.
Because income and timeshare drive everything, how those are established can significantly change the result. Our family law practice can help you understand what your guideline number is likely to look like.