This page explains general California law and the arithmetic of the statewide guideline. It is not legal advice, does not account for the facts of any particular case, and does not create an attorney-client relationship.
The formula California actually uses
Every California child support order starts from the same equation. Family Code section 4055 states the statewide uniform guideline as:
Four terms drive the result, and each one is worth understanding separately:
| CS | The guideline child support amount, per month, for one child. |
| HN | The higher-earning parent's net monthly disposable income. Note that it is the higher earner's income specifically, not the paying parent's. |
| TN | Both parents' net monthly disposable incomes added together. |
| H% | The share of parenting time the higher earner has with the child, expressed as a percentage. |
| K | A factor combining both parents' income and the higher earner's timeshare. It is not a constant, which is why two families with the same incomes can land on different numbers. |
The bracketed part of the equation, HN − (H%)(TN), is doing the real work. It measures the gap between what the higher earner brings in and what that parent already absorbs directly by having the child in their care. As the higher earner's parenting time rises, the subtracted amount rises with it and the support figure falls.
How the K factor is set
K is built in two steps. First the statute assigns a fraction based on the parents' combined net monthly income (TN). These brackets come from §4055(b)(3), as amended by SB 343, operative 1 September 2024:
| Combined net monthly income (TN) | Fraction |
|---|---|
| $0 – $2,900 | 0.165 + TN ÷ 82,857 |
| $2,901 – $5,000 | 0.131 + TN ÷ 42,149 |
| $5,001 – $10,000 | 0.250 |
| $10,001 – $15,000 | 0.10 + 1,499 ÷ TN |
| Over $15,000 | 0.12 + 1,200 ÷ TN |
Second, that fraction is multiplied by a timeshare adjustment. Where the higher earner has half the time or less, the multiplier is 1 + H%. Where that parent has more than half, it becomes 2 − H%. The two expressions meet at 1.5 when time is split evenly, so the factor moves smoothly rather than jumping at the midpoint.
A worked example, start to finish
Take two parents with two children. One nets $6,000 a month, the other nets $4,000. The higher earner has the children 30% of the time.
| Combined income | TN = $6,000 + $4,000 = $10,000 |
| Bracket fraction | $10,000 falls in the $5,001–$10,000 band, so the fraction is 0.250 |
| Timeshare multiplier | 30% is under half, so 1 + 0.30 = 1.30 |
| K | 1.30 × 0.250 = 0.325 |
| Bracket | HN − (H%)(TN) = $6,000 − (0.30 × $10,000) = $3,000 |
| One child | 0.325 × $3,000 = $975 |
| Two children | $975 × 1.6 = $1,560 per month |
The 1.6 in the last line is the multi-child multiplier from §4055(b)(4). It is not linear: two children are 1.6 times one child, three are 2, four are 2.3, and the increments keep shrinking from there. Support per child falls as the number of children rises.
The low-income adjustment
The formula has a floor built into it that most online calculators leave out. Under §4055(b)(7), where the paying parent’s net monthly disposable income is below the monthly gross of full-time minimum wage, there is a rebuttable presumption that they are entitled to a low-income adjustment.
At California’s minimum wage of $16.90 an hour (effective 1 January 2026), full time at 40 hours a week and 52 weeks a year is a threshold of about $2,929 a month. Below that, the statute allows support to be reduced by up to:
The wording is “no greater than”, so this defines a range rather than a single number: anywhere from no reduction at all down to the lowest permitted figure. The court decides where within it the order falls, weighing the §4053 principles and the effect on both parents’ net incomes. The presumption can be rebutted if the lowest permitted amount would be unjust in the particular case.
The practical effect is large. A paying parent netting $2,000 a month is about 32% below the threshold, so guideline support could be reduced by up to roughly a third. The calculator above applies this automatically: when the likely payor falls under the threshold it switches from a general range to the statutory range, from the lowest permitted figure up to the unadjusted guideline amount, which is what §4055(c) asks a computerised calculation to present.
Why parenting time moves the number so sharply
Several states use a threshold: cross a set number of overnights and a different calculation applies. California does not work that way. H% is continuous, and it appears twice in the formula, once inside the K factor and again inside the bracket. A change in parenting time therefore moves the result through two channels at once.
In the example above, raising the higher earner's timeshare from 30% to 40% lifts K to 0.350 but cuts the bracket to $2,000, bringing guideline support down to roughly $1,120 for two children. A ten-point shift in timeshare moved the figure by about 28%. This is why parenting-time percentages are so often contested, and why an estimate built on a rough guess at timeshare should be treated as provisional.
Net disposable income is not your take-home pay
The formula runs on net monthly disposable income, a statutory figure defined by Family Code section 4059. It starts from annual gross income under §4058, which is broader than wages and reaches bonuses, commissions, self-employment income, rental income and certain benefits. From that, §4059 subtracts an enumerated list: state and federal income tax liability, FICA (or an equivalent retirement contribution), mandatory union dues and retirement contributions required as a condition of employment, health insurance and state disability premiums, child or spousal support actually being paid under another court order, certain job-related expenses, and any hardship deduction under sections 4070 to 4073.
The result rarely matches the net line on a paystub. A voluntary 401(k) contribution reduces your paystub net but is not a §4059 deduction, so it does not reduce your net disposable income. The calculator above asks for your net monthly figure and uses it directly; a court would rebuild it from source documents.
Where the guideline figure and a final order diverge
The guideline number is a presumption, not a ceiling or a conclusion. Two statutes account for most of the distance between a calculated figure and a signed order.
Add-ons. Section 4062 treats childcare costs related to employment or training, and reasonable uninsured health care costs for the children, as mandatory additions. Other items, such as educational costs and travel for visitation, are discretionary. None of these are inside CS, so they are added on top of whatever the formula produces.
Rebuttal. Under section 4057, the guideline amount is presumed correct, but that presumption can be rebutted where applying it would be unjust or inappropriate in the particular case. Departures have to be justified on the record.
For a fuller treatment of how to read an estimate, what to gather before running one, and how an estimate differs from an order in practice, see our companion guide, California child support calculator: what it uses and what the estimate leaves out.
When a support order can change
A guideline figure is a snapshot of the inputs on the day it is run. Because the formula is driven by income and timeshare, a material change in either one can justify revisiting the order. California allows a party to request a modification of an existing support order when circumstances have changed materially since it was made.
The changes that move the number most are the ones the formula is most sensitive to: a job loss or a substantial raise for either parent, a shift in the parenting schedule that changes H%, a child ageing out, or a new support obligation for a child from another relationship entering the §4059 deductions. Running the calculator above with the old inputs and again with the new ones is a quick way to see whether a change is large enough to be worth pursuing.
A recalculation is not automatic. Until a court modifies the order, the existing order stands and arrears can accrue on it, so an informal agreement between parents does not by itself change what is legally owed. Our child support attorneys can advise on whether a change is likely to meet the threshold and how to bring the request.
Where this applies
The §4055 guideline is statewide, so the same formula and the same K brackets apply in every California county. What changes from courthouse to courthouse is local procedure, the way parenting-time percentages are typically documented, and how add-on expenses are presented. Mahdavi & Mahdavi handles child support matters across Ventura County and the western San Fernando Valley:
- Child support in Thousand Oaks
- Child support in Calabasas
- Child support in Agoura Hills
- Child support in Newbury Park
- Child support in Camarillo
Support rarely stands alone. It is usually decided alongside custody and the parenting schedule that sets H%, and in a dissolution it sits beside spousal support (estimate it with our California alimony calculator) and property division. Where one parent's income is variable, closely held, or tied to a business, establishing the §4059 net figure itself becomes the contested question, which is common in a high-asset divorce.
California child support: common questions
What formula does California use for child support?
CS = K[HN − (H%)(TN)], the statewide uniform guideline in Family Code §4055. Every county applies the same equation; there is no separate local formula.
What is the K factor?
A combined income-and-timeshare factor. A fraction is selected from the parents' combined net monthly income, then multiplied by 1 + H% where the higher earner has half the time or less, or 2 − H% where that parent has more.
Does California use gross or net income?
Net. Specifically net monthly disposable income under §4059, which begins with gross income under §4058 and subtracts a defined list of deductions. It is not the same as the net figure on a paystub.
Does 50/50 custody mean no child support?
No. At an even timeshare the bracket becomes HN − 0.5(TN), which is zero only if both parents earn identically. Where incomes differ, an equal timeshare still produces support flowing from the higher earner.
How is support calculated for more than one child?
The one-child result is multiplied by a factor from §4055(b)(4): 1.6 for two children, 2 for three, 2.3 for four, and progressively smaller increases above that.
Does the other parent's income matter if I am the one paying?
Yes. Both incomes enter through TN, and the combined figure also selects the K bracket. A change in either parent's income can move the result.
What if the paying parent earns very little?
Section 4055(b)(7) raises a rebuttable presumption of a low-income adjustment where the payor’s net monthly disposable income is below the monthly gross of full-time minimum wage, about $2,929 at the 2026 rate. The reduction is capped by a statutory fraction, and the calculator above switches to that range automatically when it applies.
How accurate is this calculator?
It implements §4055 faithfully and reproduces the statute's own worked example. Its accuracy for your case depends entirely on your inputs, particularly whether your net figure is a true §4059 net disposable income and whether your timeshare percentage reflects an actual parenting schedule. It does not apply add-ons under §4062 or any §4057 rebuttal.