How California calculates child support

Most U.S. states use an Income Shares model — they combine both parents' incomes, look up a base amount from a schedule table, and split the result proportionally. California does not. Instead, it uses a single algebraic equation that factors in both parents' net disposable income (income after taxes and deductions) and the high earner's custody time-share.

Net income is computed from gross using each parent's tax situation — filing status, dependents, FICA, and allowable deductions per § 4059. No schedule table is involved.

Each variable defined:

Sign rule (§ 4055(b)(5)): If CS > 0, the high earner pays the lower earner. If CS < 0, the lower earner pays the higher earner. This handles the case where the higher earner also has significantly more custody time.

California's K-factor hadn't been updated since 1992 — until SB 343, which took effect September 1, 2024.

The change: the formula for K when the high earner has more than 50% custodywas corrected to be symmetric. Before SB 343:

After SB 343 (post-September 1, 2024):

The practical effect: at any given H%, the K-factor is now symmetric around 50%. A parent with 20% custody produces the same K as one with 80% custody. Most online calculators still use the pre-2024 formula and will overstate support when the high earner has more than 50% custody.

California computes child support from net income, not gross. Each parent's gross income is reduced by the deductions listed in § 4059:

• § 4059(a) — Federal and California income taxes, based on filing status and dependents
• § 4059(b) — FICA (Social Security 6.2% + Medicare 1.45%), or self-employment tax (15.3% on 92.35% of income)
• § 4059(c) — Mandatory union dues and retirement contributions required as a condition of employment
• § 4059(d) — Health and disability insurance premiums for children; CA SDI
• § 4059(e) — Other court-ordered child or spousal support paid to non-parties

This is why two parents with the same gross income can have very different child support obligations — a single filer with no dependents pays significantly more tax than a head-of-household filer with multiple dependents.

The K-factor converts raw net income into an allocation for child support. It has two parts: a base fraction that scales with combined income (TN), and a multiplier that adjusts for the high earner's custody time (H%).

The fraction(TN) function is a piecewise formula from § 4055(b)(3):

Example from the statute: fraction(1,000) ≈ 0.177; K(H% = 20%, TN = $1,000) ≈ 0.21.

For more than one child, the one-child support amount is multiplied by a factor from § 4055(b)(4):

Per § 4055(b)(8), the court may also allocate support per child: the first child's share equals the one-child amount; the second child's share equals the difference between the two-child and one-child totals; and so on.

A rebuttable presumption applies when the obligor's net monthly income is below the amount earned working full-time at California minimum wage ($16.50/hr × 40 hrs × 52 wks ÷ 12 ≈ $2,860/month in 2025).

The maximum reduction is:

Courts may apply a smaller reduction than the maximum if the standard adjustment would be unjust or inappropriate. The adjustment is in the obligor's favor, not the obligee's.

When the obligor earns significantly more than average, courts may deviate downward from the guideline if the formula amount exceeds the children's actual needs. This is a discretionary deviation — the guideline amount is still presumed correct; the high-earning parent must overcome that presumption.

Our calculator flags a warning when the obligor's gross monthly income exceeds $20,000.

• Hardship deductions (§§ 4070–4073) — catastrophic losses, minimum basic needs of additional children
• Job-related expenses as court discretion deductions (§ 4059(f))
• Itemized tax deductions — calculator always uses the standard deduction
• Federal Child Tax Credit phase-out at higher income levels
• Different timeshare percentages for different children
• Imputed income for voluntarily unemployed or underemployed parents
• Agreement-based deviations and judicial discretion generally

• Cal. Fam. Code § 4055 — the guideline formula →
• Cal. Fam. Code § 4059 — net disposable income →
• Cal. Fam. Code § 4057 — deviation factors →
• SB 343 (2024) — K-factor update →
• Official DCSS Guideline Calculator →