California uses a precise algebraic formula to calculate child support. The guideline formula considers both parents’ net disposable income, the percentage of time each parent has primary physical responsibility for the children, and statutory K-factor brackets that determine how much combined income is allocated to child support.
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California calculates child support using the Guideline formula (algebraic) established under FC §4055. This is a statutory formula that leaves limited room for judicial discretion on the base calculation amount, ensuring consistency and predictability for families across the state.
CS = K[HN - (H%)(TN)] where K = amount of both parents’ income to allocate for child support (determined by K-factor bracket tables), HN = high earner’s net disposable income, H% = approximate percentage of time the high earner has primary physical responsibility, TN = total net disposable income of both parents.
The calculation begins with determining each parent's net disposable income, which includes virtually all sources of income minus taxes, mandatory payroll deductions, and certain hardship deductions. The court then applies the statutory formula using the time each parent has primary physical responsibility for the children and the applicable income allocation brackets.
Beyond the base formula amount, Californiarequires additional contributions for mandatory add-on expenses. These typically include childcare costs necessary for the custodial parent's employment or education, uninsured healthcare expenses for the children, and in some cases, educational and travel costs. These add-ons are split between parents in proportion to their respective incomes.
Primary statute: FC §4055
The guideline formula is CS = K[HN - (H%)(TN)], where K is the income allocation factor, HN is the high earner’s net disposable income, H% is the high earner’s custodial time share, and TN is the total net disposable income of both parents.
K-factor brackets determine the percentage of combined income allocated to child support, updated by SB 343 effective September 2025.
Multi-child multipliers adjust the base amount: 1.0 for one child, 1.6 for two, 2.0 for three, 2.3 for four, 2.5 for five, up to 2.86 for ten or more children.
A low-income safe harbor protects obligors earning below $2,929 per month from guideline calculations that would reduce their income below subsistence.
Mandatory add-ons for childcare costs and health insurance premiums are split proportionally by income after the base support amount is determined.
Courts may deviate from the guideline amount only in limited circumstances enumerated in FC §4057.
California’s formula is one of the most mathematically precise in the nation — no judicial discretion on the base calculation.
Net disposable income is defined by FC §4059 and includes virtually all income sources minus taxes, mandatory deductions, and hardship deductions.
The DissoMaster and XSpouse software programs are commonly used by attorneys and courts to run the guideline formula.
Add-on expenses (childcare, uninsured health costs, educational needs, travel for visitation) are addressed separately from the base formula.
SB 343 (effective September 2025) updated the K-factor brackets used in the guideline formula, adjusting income allocation percentages for the first time since 2013.
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Residency Requirement
6 months state residency + 3 months county residency
Mandatory Waiting Period
6 months from date of service of the petition
No-Fault Ground
Irreconcilable differences (FC §2310)
State Income Tax
Progressive 1% to 12.3%, plus 1% Mental Health Services Act surcharge on income above $1,000,000
California uses the Guideline formula (algebraic) under FC §4055. California uses a precise algebraic formula to calculate child support. The guideline formula considers both parents’ net disposable income, the percentage of time each parent has primary physical responsibility for the children, and statutory K-factor brackets that determine how much combined income is allocated to child support.
Key factors include both parents' income, the number of children, custodial time share, mandatory add-on expenses (childcare, health insurance), and any applicable deductions. The guideline formula is CS = K[HN - (H%)(TN)], where K is the income allocation factor, HN is the high earner’s net disposable income, H% is the high earner’s custodial time share, and TN is the total net disposable income of both parents.
Yes. Either parent can request a modification if there has been a material change in circumstances, such as a significant change in income, custody arrangements, or the needs of the child. The court will recalculate support using the same statutory formula.
CS = K[HN - (H%)(TN)] where K = amount of both parents’ income to allocate for child support (determined by K-factor bracket tables), HN = high earner’s net disposable income, H% = approximate percentage of time the high earner has primary physical responsibility, TN = total net disposable income of both parents.
Get a personalized child support calculation based on California's actual statutory formula.
Start My Free CaliforniaEstimate →This tool provides educational estimates based on California family law and does not constitute legal advice. Child support calculations are approximations of the statutory formula and may differ from court-ordered amounts. Consult a licensed California family law attorney before making decisions. Primary statute: FC §4055.