How Child Support Is Calculated in California

California uses a single statewide formula, Family Code §4055, to calculate guideline child support. Every court in the state uses this formula. The result is presumptively correct, meaning the court must order this amount unless a specific reason to deviate is established under FC §4057.

This page explains each step of the calculation.

The formula

CS is the monthly child support amount. K is the income allocation factor, which varies by combined income and parenting time. HN is the higher earner's net disposable income (after taxes). H% is the higher earner's parenting time (timeshare %). TN is the combined net disposable income of both parents.

In plain terms: the formula looks at how much each parent earns after taxes, how much time each parent spends with the child, and then allocates a portion of the income difference as support. The parent who earns more and spends less time with the child pays. The amount depends on how large the income gap is and how the parenting time is split.

Step 1: Determine gross income

FC §4058(a) defines gross income broadly. It includes salary, wages, bonuses, commissions, rental income, dividends, pensions, Social Security, unemployment benefits, disability insurance, self-employment income (after business expenses), and essentially all other income from any source. If a parent is voluntarily unemployed or underemployed, the court may impute income based on earning capacity under FC §4058(b).

Step 2: Compute net disposable income

FC §4059 defines net disposable income as gross income minus:

• Federal income tax (using actual marginal brackets, not a flat rate)
• California state income tax (separate brackets from federal)
• Social Security tax (6.2% up to the wage base)
• Medicare tax (1.45% plus 0.9% additional above the threshold)
• California SDI
• Health insurance premiums (self and children)
• Union dues
• Existing child or spousal support orders from other cases
• Hardship deductions (extraordinary health expenses, catastrophic losses)

Pre-tax retirement contributions like a 401(k) reduce net disposable income indirectly by lowering taxable income, which reduces the tax amounts subtracted above. They are not subtracted a second time.

This step is where most of the complexity lives. SupportSplit computes taxes using real marginal brackets, handles the SALT cap with the OBBBA phasedown, compares standard vs. itemized deductions separately for federal and California, and treats HSA contributions as deductible on the federal return but not the California return.

Step 3: Compute the K factor

K is computed in two steps.

First, a base fraction (sometimes called the K ratio) is looked up from a statutory table based on the combined net disposable income (TN) of both parents. The table has five income bands, updated by SB 343 (effective September 2024):

• $0 to $2,900: K ratio = 0.165 + TN / 82,857
• $2,900 to $5,000: K ratio = 0.131 + TN / 42,149
• $5,000 to $10,000: K ratio = 0.25 (flat)
• $10,000 to $15,000: K ratio = 0.10 + 1,499 / TN
• Above $15,000: K ratio = 0.12 + 1,200 / TN

Then, the K ratio is multiplied by a parenting time adjustment:

• If the higher earner has 50% or less parenting time: adjustment = 1 + H%
• If the higher earner has more than 50%: adjustment = 2 - H%

The final K = K ratio × parenting time adjustment.

This two-step structure is what makes the relationship between parenting time and support non-linear. The parenting time adjustment amplifies the K ratio as parenting time approaches 50/50.

Step 4: Apply the formula

With K, HN, H%, and TN computed, the formula CS = K × [HN − (H%)(TN)] produces the base support amount for one child.

For multiple children, the base is multiplied by a statutory factor under FC §4055(b)(4):

• 1 child: 1.0×
• 2 children: 1.6×
• 3 children: 2.0×
• 4 children: 2.3×
• 5 children: 2.5×

These multipliers reflect that many child-rearing costs are shared across siblings. Two children do not cost exactly twice as much as one.

How parenting time affects support

The relationship between custody time and support is not linear. At lower parenting time percentages (say, 10 to 20%), small increases have a modest effect on the support amount. As parenting time approaches 50/50, the same percentage increase has a much larger impact. This happens because the K factor itself grows as parenting time approaches equal, amplifying the effect.

The chart below shows guideline support for a sample scenario: Party A earns $10,000/month, Party B earns $4,000/month, 1 child, both filing MFS with standard deductions. The horizontal axis is Party A's parenting time percentage.

The other major driver is the income gap. Support is primarily a function of the difference in net disposable incomes. When incomes are similar, support is low regardless of parenting time. When they diverge, parenting time becomes the main lever.

Try it with your own numbers. The calculator updates this chart in real time as you adjust parenting time, income, and deductions.

Step 5: Low-income adjustment

If the obligor's net disposable income falls below $2,929/month (in 2026, derived from California minimum wage at 40 hours/week), the court may reduce guideline support proportionally under FC §4055(b)(7). This protects low-income obligors from support orders that would leave them below subsistence.

Step 6: Add-on expenses

On top of the base guideline amount, FC §4062 provides for additional child-related costs to be split between parents. Mandatory add-ons include work-related childcare and uninsured health care. Discretionary add-ons include educational expenses, extracurricular activities, and travel for visitation. These costs are allocated in proportion to each parent's adjusted net income (adjusted by the support amount per FC §4061(c)).

What the calculator includes

SupportSplit models the following for each parent, starting from monthly gross income:

• Federal income tax. 2026 marginal brackets (7 brackets, 10% to 37%), OBBBA/TCJA rates.
• California income tax. 2026 marginal brackets (10 brackets, 1% to 13.3%), including mental health services surcharge above $1M.
• Standard vs. itemized deductions. Auto-selected separately for federal and California.
• SALT cap. $40,400 base cap per OBBBA (2026), phasing down for MAGI above $505,500.
• Social Security, Medicare, and California SDI.
• Federal Child Tax Credit and California credits. With phaseouts.
• Pre-tax deductions. 401(k), IRA, mandatory retirement, dependent care FSA, HSA (federal only).
• Post-tax deductions. Health insurance, union dues, existing support orders, hardship.
• Above-the-line adjustments. Student loan interest, educator expenses, self-employment tax, pre-2019 alimony.
• SB 343 K-factor bands and child multipliers.
• Low-income adjustment and add-on expense allocation.

What we don't include

No calculator captures every possible adjustment. SupportSplit does not currently account for:

• Non-wage income requiring special treatment, such as partnership K-1 income, stock options, and deferred compensation.
• Self-employment tax beyond the above-the-line deduction for half of SE tax.
• Imputed income determinations, which require judicial findings under FC §4058(b).
• Departures from guideline under FC §4057, such as extraordinary high income or special needs.

For situations involving these factors, consult a licensed California family law attorney.