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California child support: the K formula, timeshare, and the SB 343 changes

California has no support table. Family Code section 4055 is an equation that uses both parents' net disposable incomes and the higher earner's share of parenting time, so every extra day with the children moves the number. SB 343 rewrote that equation for orders from September 1, 2024. This guide walks through it with real arithmetic.

Estimate free

At a glance

FormulaCS = K[HN - (H%)(TN)], Cal. Fam. Code 4055, as rewritten by SB 343 (operative September 1, 2024)
Income baseEach parent's net monthly disposable income: gross income minus actual taxes, FICA, mandatory dues and retirement, health premiums, and support paid for others
TimeshareBuilt into the formula as H%, the high earner's share of time; no threshold or cliff
Low-income adjustmentPresumed when the payer's net income is below full-time state minimum wage earnings
Mandatory add-onsWork or training child care and reasonable uninsured health costs, split by net income unless the court orders otherwise
Support endsAt 18, or for a full-time high school student who is not self-supporting, at the end of 12th grade or 19, whichever comes first
AgencyCalifornia Department of Child Support Services and the local child support agencies (LCSAs)

The pieces of K[HN - (H%)(TN)]

HN is the higher earner's net monthly disposable income and TN is both parents' combined. H% is the approximate percentage of time the higher earner has primary physical responsibility for the children; when children have different schedules, it is the average. K is the share of combined income allocated to the children: a fraction that depends on TN, multiplied by one plus H% when the higher earner has the children half the time or less, or by two minus H% when they have more.

SB 343 replaced the old income bands. Today the fraction is 0.165 + TN/82,857 up to $2,900 of combined net income, 0.131 + TN/42,149 from $2,901 to $5,000, a flat 0.25 from $5,001 to $10,000, 0.10 + 1,499/TN from $10,001 to $15,000, and 0.12 + 1,200/TN above that. The result is for one child; for more, multiply by 1.6 for two, 2 for three, 2.3 for four, 2.5 for five, rising to 2.86 for ten. A positive result means the higher earner pays; a negative one means the lower earner pays the higher earner.

Net disposable income starts from gross income and subtracts the state and federal income tax actually payable for the parent's real filing status, FICA (or its equivalent), mandatory union dues and retirement contributions, health insurance premiums for the parent and children they must support, state disability insurance, and child or spousal support actually paid for someone outside this case. A new spouse's or partner's income is not counted, except in an extraordinary case of extreme and severe hardship to a child.

Worked example: California

  1. Parent A, the higher earner, has $7,000 a month in net disposable income; Parent B has $3,000. TN is $10,000, which falls in the $5,001 to $10,000 band, so the fraction is 0.2500.
  2. Parent A has the children 20% of the time. K is (1 + 20%) times 0.2500, or 0.3000.
  3. The bracket is HN minus H% of TN: $7,000 minus 20% of $10,000. Multiplying by K gives the one-child amount, and the two-child factor is 1.6.
  4. Result: Parent A pays $2,400 a month in guideline support for the two children, about $28,800 a year, before child care and uninsured health costs are added.
  5. If Parent A's timeshare rose to 40%, the same incomes would produce about $1,680 a month.

This is an estimate, not legal advice. Figures are rounded to the dollar and leave out the insurance, childcare, and other adjustments your case may have. Try your own numbers in CoreParent's calculator, then confirm with the state's official calculator.

Timeshare in California: every percentage point counts

Because H% sits inside the formula twice (it scales K and it reduces the bracket), there is no overnight threshold where support suddenly drops. In the example, Parent A's time going from 20% to 40% lowers support from $2,400 to about $1,680, and at an even 50/50 split it is still about $1,200 a month, because Parent A earns 70% of the family's net income. Support reaches zero only when the parents' incomes and time line up so the bracket is zero.

H% is the time the higher earner is actually expected to have primary physical responsibility, usually figured from the parenting plan. In a default, or when a party fails to appear and there is no evidence of the schedule, the statute sets H% at zero if the noncustodial parent is the higher earner. That is why a written parenting schedule matters for support as much as for custody.

Child care and uninsured health costs on top

Section 4062 requires the court to order two add-ons: child care actually incurred for employment or reasonably necessary job training, and reasonable uninsured health care costs. The court may also add costs for a child's educational or other special needs and travel expenses for visitation. Since September 2024, these are divided in proportion to the parents' net disposable incomes (adjusted for any spousal support) unless a party asks for, or the court chooses, a different split. The add-ons are calculated after the basic formula amount.

The low-income adjustment

When the paying parent's net disposable income is less than what a full-time worker earns at the California minimum wage (40 hours a week, 52 weeks a year), there is a rebuttable presumption that the parent gets a low-income adjustment. The reduction is capped at the support amount times the gap between the minimum wage income and the parent's net income, divided by the minimum wage income. A court may refuse it if applying the lowest permitted amount would be unjust, considering both households' net incomes. Computer programs used by courts must show the allowed range.

When support ends and how to change it

The duty to support continues past 18 for an unmarried child who is a full-time high school student and not self-supporting, until the child finishes 12th grade or turns 19, whichever comes first. A documented medical condition can excuse the full-time requirement, and parents may agree to support beyond that.

To change an order, a parent files a request with the court, for example after an income change or a new parenting schedule, and the judge can change the amount only as far back as the date the request was filed. Unpaid support is a money judgment, and California's general rate on unsatisfied money judgments is 10% a year. Either parent can open a case with the local child support agency, part of the Department of Child Support Services, which also publishes the official Guideline Calculator.

Documents you will need

California child support FAQ

What is the California child support formula?

CS = K[HN - (H%)(TN)], from Family Code 4055. HN is the higher earner's net income, TN is both parents' net income, H% is the higher earner's timeshare, and K depends on TN and H%. Multiply by 1.6 for two children, 2 for three.

What changed in California child support in 2024?

SB 343 took effect for calculations from September 1, 2024. It set new K income bands, rewrote the low-income adjustment around full-time minimum wage earnings, and made child care and uninsured health costs split by net income unless the court orders otherwise.

How much is child support with 50/50 custody in California?

It depends on the income gap. At equal time, K uses 1.5 times the fraction and the bracket is HN minus half of TN. In our example, where one parent earns 70% of the combined net income, that is about $1,200 a month for two children.

Does a new spouse's income affect child support in California?

Not ordinarily. Family Code 4057.5 bars counting a new spouse's or nonmarital partner's income for either parent, except in an extraordinary case where leaving it out would cause extreme and severe hardship to a child.

Is there an official California child support calculator?

Yes. The Department of Child Support Services publishes the California Guideline Calculator, built on the same guideline the courts use. It gives an estimate; the commissioner or judge sets the order.

Next steps

Sources

Verified against these official sources in October 2026:

Child support in every other state

All states

This is general educational information, not legal advice, and official figures change. Last verified: October 2026. Consult a licensed family-law attorney in California.