SupportSplit's Guide to California Child Support

California uses a statewide formula, Family Code §4055, to calculate guideline child support. The formula produces a presumptively correct amount based on each parent's income, the custody parenting time, and the number of children.

SupportSplit implements this formula with real 2026 federal and California tax brackets. In our internal testing, results for common W-2 scenarios with standard deductions have been within 5% of Judicial Council-certified tools. Our calculator has not been submitted for Judicial Council certification and has not been independently verified by a CPA or legal expert.

The formula

CS is the 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.

K is computed from income band tables updated by SB 343 (effective September 2024). For multiple children, the result is multiplied by a statutory factor: 1.6× for 2 children, 2.0× for 3, 2.3× for 4, and 2.5× for 5.

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, which controls how income is allocated between parents, is itself a function of both the parenting time split and the combined income level.

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. Because the formula subtracts H% × TN (the higher earner's share of combined income weighted by their parenting time), 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. For a step-by-step walkthrough of every part of this formula, see How Child Support Is Calculated in California.

Why free?

The guideline formula is public law. The tax brackets are public data. We think every parent, not just those who can afford an attorney, should be able to see what child support looks like for their situation, check the math, and walk into a mediation or courtroom informed.

Can I use this in court?

SupportSplit is not a Judicial Council-certified calculator, so a court will not accept our output as an official guideline calculation. The court will rely on a certified tool for the final order.

That said, understanding the number before you walk into court or mediation is genuinely useful. You can use SupportSplit to:

• See what guideline support looks like for your income and parenting time before your first attorney consultation, so you're not hearing the number for the first time in someone's office.
• Model different scenarios. What if income changes? What if parenting time shifts from 20% to 35%? What if one parent adds a 401(k) contribution? Understanding how the inputs affect the output helps you ask better questions.
• Evaluate whether what's being proposed in mediation or settlement makes sense relative to guideline. If someone proposes a number significantly above or below guideline, you'll know it and can ask why.

The goal is not to replace your attorney or the court's calculation. It's to make sure you understand the math well enough to participate meaningfully in your own case.

How the calculator is built

The calculation engine runs entirely in your browser. Nothing is sent to a server. Your income, parenting time, and deduction inputs never leave your device. There are no accounts, no cookies, and no way for us to see what you enter. Tax brackets and rates are updated annually when the IRS and California Franchise Tax Board publish new figures.

What we calculate

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

• Federal income tax. 2026 marginal brackets (7 brackets, 10% to 37%), with the OBBBA/TCJA rates made permanent.
• California income tax. 2026 marginal brackets (10 brackets, 1% to 13.3%), including the mental health services surcharge above $1M.
• Federal standard deduction. Selected by filing status, or itemized deductions when they produce a greater benefit.
• California standard deduction. Separate from federal, also auto-selected vs. itemized.
• Itemized deductions. Mortgage interest, property tax, state tax, and charitable contributions, with automatic standard-vs-itemized comparison.
• SALT cap. $40,400 base cap per OBBBA (2026), phasing down to $10,000 for MAGI above $505,500. California has no state-level SALT cap, and the calculator handles this correctly.
• Social Security tax. 6.2% up to the $176,100 wage base.
• Medicare tax. 1.45% base rate plus 0.9% additional Medicare tax above the filing-status threshold.
• California SDI. 1.1% up to the $174,400 wage base.
• Federal Child Tax Credit. $2,200 per child with phaseout above $200K/$400K (OBBBA).
• California personal and dependent credits. With AGI-based phaseout.
• Pre-tax deductions. 401(k)/403(b)/TSP, IRA, mandatory retirement, dependent care FSA, and HSA. HSA is a federal-only deduction and is correctly excluded from California taxable income.
• Post-tax deductions. Health insurance (self and children), union dues, existing child support orders, and hardship deductions under FC §4070 through §4073.
• Above-the-line adjustments. Student loan interest, educator expenses, self-employment tax, and pre-2019 alimony.
• SB 343 K-factor bands. The five statutory income bands that determine the allocation factor.
• Child multipliers. FC §4055(b)(4) statutory factors for 1 through 5 children.
• Low-income adjustment. Automatic adjustment when the obligor's net falls below $2,929/month (derived from CA minimum wage).
• Add-on expenses. Childcare, children's health insurance, educational and special needs, and visitation travel, allocated proportionally between parents per FC §4062.

What we don't calculate

No calculator captures every possible adjustment. We do not currently account for:

• Non-wage income sources that require 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.

What's different about SupportSplit

Here is what SupportSplit does under the hood.

• Real marginal tax brackets. Federal and California taxes are computed bracket by bracket, the same way the IRS and FTB do.
• Current SALT cap with phasedown. The 2026 SALT cap is $40,400 with a 30% phasedown above $505,500 MAGI, not the flat $10,000 from prior years.
• Automatic itemized vs. standard comparison. The calculator determines which deduction method produces the lower tax liability for each parent, separately for federal and California returns.
• HSA handled correctly. HSA contributions are deductible on the federal return but not on the California return.
• Full formula breakdown. Every tax bracket, every deduction, and every step of the guideline formula is displayed.

The interface

The calculator interface is designed to keep the result visible and the inputs easy to adjust.

• Pinned result card. The support amount stays visible as you scroll through inputs and the formula breakdown, so you always see how changes affect the number.
• Real-time updates. Every slider and input updates the result instantly. There's no "calculate" button.
• Income sensitivity analysis. See how the support amount changes across a range of incomes. Useful for understanding what a raise, job loss, or return to work would mean.
• Support timeline. Enter children's ages to see exactly when support steps down and ends as each child turns 18 (or 19 if still in high school per FC §3901).

Judicial Council-certified calculators

SupportSplit is not a Judicial Council-certified calculator. For court filings, you may need a result from a certified tool. The Judicial Council certifies guideline calculators under Family Code §3830. The following are currently certified for 2025 through 2026:

• Xspouse. Web-based and desktop calculator used by family law practitioners.
• Family Law Software. Comprehensive family law practice software with guideline support calculations.
• CalSupport / CalSupport Pro. Desktop guideline calculator software.
• FamilySoft SupportCalc. Part of the FamilySoft suite for family law practitioners.

The Judicial Council maintains the official list at courts.ca.gov. SupportSplit uses the same public formula (FC §4055) and public tax data as these tools, but has not been submitted for certification. We'd recommend using SupportSplit for understanding, planning, and scenario modeling, and a certified tool when you need a result for court.

Who built this

Support Split LLC is a Delaware company that builds financial tools for family law. We are not affiliated with any law firm, court, or government agency.

Questions, corrections, or feedback: info@supportsplit.com