California uses a statewide formula to calculate child support — one that every court in the state must follow in almost every case. The formula looks complicated at first glance, but it really boils down to two things: how much each parent earns after taxes, and how much time each parent spends with the child. This guide breaks it down in plain English, including what changed in 2025 under Senate Bill 343 — the most significant update to California's child support rules in more than 30 years.
- How California's guideline formula works — and what each piece means
- What "net disposable income" is and how it's calculated
- How parenting time (timeshare) affects the support amount
- What "add-on" expenses are and how they're split
- What changed under SB 343 in 2025
- How the low-income adjustment works
- A plain English worked example with real numbers
The Formula — What It Is and What It Means
California uses a single statewide guideline formula for child support. Courts are generally required to follow it — departing from it requires specific findings that explain why the guideline amount would be unjust or inappropriate in a particular case.
The formula is:
In plain English: the formula calculates how much money — after taxes — is available between both parents, determines what share of that should go toward the child, and adjusts based on how much time each parent spends with the child.
Two inputs do most of the work: net income and parenting time. The K-factor applies a multiplier to the result. Each of these is explained below.
Step 1: Calculate Net Disposable Income
California doesn't use gross income (your total paycheck before deductions) in the child support formula. It uses net disposable income — what you actually take home after required deductions. As of September 2025, SB 343 updated the K-factor bands to use net income rather than gross income for the first time since the formula was adopted in 1992.
Net disposable income is calculated by starting with gross income and subtracting mandatory deductions. What counts as a deduction:
- Federal and state income taxes (based on your filing status)
- Social Security and Medicare (FICA) contributions
- State disability insurance (SDI)
- Health insurance premiums you pay for yourself (not the child's portion)
- Mandatory union dues
- Required retirement contributions (such as pension contributions that are not optional)
- Child support orders for children from other relationships
Step 2: The K-Factor — What Percentage Goes to Child Support
The K-factor determines what share of the parents' combined net income is devoted to child support. It's a percentage that varies based on income level — the idea is that higher-income families spend a larger absolute amount on children, but a smaller relative share of their income.
Under SB 343, the K-factor bands were updated to reflect net income rather than gross income and to reflect current wage levels. The lowest band now starts at approximately $2,900 per month in combined net income — corresponding to full-time work at California's $16.90 minimum wage as of January 2026.
The K-factor also increases for more than one child. California applies statutory multipliers: for two children, the K-factor is multiplied by 1.6; for three children, by 2.0; and so on. A case with two children produces roughly 60% more support than a comparable single-child case.
Step 3: Timeshare — How Parenting Time Affects Support
The H% variable — the higher-earning parent's percentage of time with the child — has an enormous effect on the support calculation. As the higher-earning parent's timeshare increases, their support obligation generally decreases. This reflects the logic that more time with a child means more direct spending on that child.
Timeshare is measured as a percentage of the year. Courts typically count overnights — 365 overnights total, one parent has some number of them, and that number divided by 365 gives the timeshare percentage.
| Common custody arrangement | Approximate overnights | Timeshare % |
|---|---|---|
| One parent has full custody | 0 overnights for higher earner | 0% |
| Every other weekend + some holidays | ~73–90 overnights | ~20–25% |
| Alternating weekends + one weeknight | ~109 overnights | ~30% |
| 50/50 shared custody | 182–183 overnights each | 50% |
| One parent has most of the time | ~274 overnights for primary parent | 75% (primary) |
Because the formula uses the higher earner's timeshare percentage (H%), a higher-earning parent who has 50% custody will generally pay significantly less support than one who has 20% custody — even if their income is identical.
Step 4: Add-On Expenses — On Top of the Guideline Amount
The guideline support amount covers the child's basic living costs — shelter, food, clothing, routine expenses. But two categories of expense are added on top as separate line items under California Family Code Section 4062.
Mandatory add-ons — courts are required to add these when applicable:
- Work-related childcare costs (daycare, after-school care paid so a parent can work or attend job training)
- Uninsured medical and dental expenses for the child
Discretionary add-ons — courts have the option to add these based on the circumstances:
- Educational costs, tuition, or special needs expenses
- Travel expenses for visitation
- Other extraordinary costs the court finds reasonable
The Low-Income Adjustment
California provides a special adjustment for parents whose net disposable income falls below full-time minimum wage. As of January 2026, the low-income threshold is approximately $2,929 per month — equivalent to full-time work at the state minimum wage of $16.90 per hour.
When the paying parent's net income is below this threshold, a rebuttable presumption arises that the standard guideline amount would cause financial hardship. The court may reduce the support amount — though it is still required to consider the child's needs. This adjustment has always existed in California law; SB 343 updated the threshold to reflect current wage levels.
How Courts Calculate the Actual Number — XSpouse
In practice, attorneys and judges don't run the formula by hand. California courts use certified software to calculate the guideline amount. As of April 2025, the standard software is XSpouse, which replaced DissoMaster — the previously dominant tool that had been in use since the 1990s.
XSpouse applies all the SB 343 updates, runs the tax calculations, and produces the presumptive guideline number based on both parties' financial information. The output of this software is typically treated as the starting point for any child support discussion — it's the number both attorneys and the court typically reference.
California also maintains a free public guideline calculator at childsupport.ca.gov, which can give you a rough estimate based on your inputs. The Know Your Half child support calculator also covers California using the current formula.
Worked Example
In this example, Parent A earns $7,500/month in net disposable income and Parent B earns $3,000/month in net disposable income. They have one child. Parent A (the higher earner) has the child approximately 30% of the year — about 109 overnights.
Applying the formula: CS = K [ HN − (H%)(TN) ] = 0.25 × [$7,500 − (0.30)($10,500)] = 0.25 × [$7,500 − $3,150] = 0.25 × $4,350 = approximately $1,088/month in base guideline support.
On top of this, the parties share a monthly childcare cost of $900. Under SB 343, this is split proportionally: Parent A earns 71% of combined income ($7,500 ÷ $10,500), so Parent A's share of childcare is approximately $639/month and Parent B's share is $261/month.
This is a simplified illustration. Actual calculations depend on the specific K-factor band applicable to the combined net income, exact tax deductions for each parent, and any other adjustments the court applies. Always use certified software or consult a licensed family law attorney for an accurate calculation in your case.
Can Child Support Be Different From the Guideline Amount?
Courts are permitted to depart from the guideline in limited circumstances — and they're required to state specific reasons on the record when they do. Common situations where a court may consider a departure include cases where the guideline amount would be unjust given the child's particular needs, cases where one parent's income is unusually high and the guideline amount would exceed the child's reasonable needs, and cases where the parties agree in writing to a different amount — provided the court finds the agreement is in the child's best interests and both parents are fully informed.
Modifying Child Support After It's Set
Child support in California is not permanent in the way a divorce decree is. Either parent may request a modification when there has been a material change in circumstances — meaning something significant has changed that would produce a meaningfully different guideline amount if recalculated today.
Common reasons for modification include a significant change in either parent's income (job loss, promotion, new job), a change in parenting time, a change in the child's needs (new medical condition, special educational requirements), or one parent becoming responsible for additional children from another relationship.
The modification process involves filing a motion with the court, providing updated financial information, and having the guideline recalculated. Courts apply the same formula using the current figures. Support can be modified upward or downward depending on the new calculation.
See how California child support estimates apply to your numbers.
The Know Your Half child support calculator covers California using the current SB 343 formula — enter both parents' incomes, custody split, and add-ons for a plain English estimate. Free, no login required.
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