California Family Code 4055: Formula, Inputs, and Adjustments

California Family Code 4055 sets the statewide formula every court uses to calculate guideline child support: CS = K[HN − (H%)(TN)]. The formula takes the higher earner’s net monthly income, subtracts the share of combined family income that corresponds to the time that parent already spends with the children, and multiplies the result by an income-tier factor to produce a presumptively correct monthly support amount.¹ Senate Bill 343, effective September 1, 2024, revised the income tiers and the low-income adjustment, so recent orders tend to run slightly higher than those calculated under the earlier version.

What Each Letter in the Formula Means

The equation looks algebra-heavy, but each variable stands for something ordinary.

CS is the monthly support the court orders.
K is the fraction of combined parental income allocated to child support, pulled from a tiered table and then adjusted for custodial time.
HN is the higher-earning parent’s net monthly disposable income.
H% is the approximate share of time the higher earner has primary physical responsibility for the children.
TN is the combined net monthly disposable income of both parents.

Read in plain English, the bracketed portion asks how much of the higher earner’s income is not already being spent on the children during that parent’s custodial time. The more time the higher earner has, the smaller the gap, and the smaller the support payment, because that parent is already covering costs directly.

Courts do not do this math by hand. They run the inputs through Judicial Council–certified software: Cal Support (and CalSupport Pro), Family Law Software, FamilySoft SupportCalc, or Xspouse.² Cases handled by a local child support agency must use the California Child Support Guideline Calculator. The programs carry current tax tables and standard deductions, which keeps the net income figures accurate. The number the software produces is presumed correct, and a judge who orders something different has to put specific reasons on the record.

How K Is Built

K is where the formula gets its character. It is not a flat percentage; it depends on the parents’ combined net monthly income (TN) and the higher earner’s time-share (H%). The statute sets five income tiers, each with its own base fraction.¹

TN of $0 to $2,900: 0.165 + TN ÷ 82,857
TN of $2,901 to $5,000: 0.131 + TN ÷ 42,149
TN of $5,001 to $10,000: 0.250 (flat)
TN of $10,001 to $15,000: 0.10 + 1,499 ÷ TN
TN over $15,000: 0.12 + 1,200 ÷ TN

Once you have the right fraction for the family’s income tier, you multiply it by (1 + H%) when the higher earner has 50 percent or less of the parenting time, or by (2 − H%) when the higher earner has more than 50 percent. The product is K.

A worked example makes it concrete. If TN is $1,000 per month and the higher earner has 20 percent of the custodial time, K equals (1 + 0.20) × (0.165 + 1,000 ÷ 82,857), which comes out to roughly 0.21.¹ The SB 343 revisions nudged these tiers and fractions upward across most of the income range, which is why orders issued after September 2024 generally come out a bit higher than they would have under the old version.

What Counts as Income

The formula only works if HN and TN are right, and those numbers start with gross income. Family Code 4058 defines gross income broadly: wages, salaries, commissions, bonuses, royalties, rental income, dividends, pensions, interest, trust income, annuities, workers’ compensation, unemployment and disability benefits, Social Security benefits, severance pay, and veterans’ benefits that are not need-based.³ Military housing and food allowances count, and so does spousal support received from someone who is not a party to the current case.

For a business owner, gross income means total receipts minus legitimate operating expenses. The court can also count employee benefits and self-employment perks when those benefits reduce the parent’s own living costs. The rule of thumb: if money or its equivalent flows to the parent and they can use it, the court can probably count it.

When Income Gets Imputed

A parent who quits work or cuts hours to shrink a support obligation does not get the benefit of their reduced paycheck. Under Family Code 4058(b), the court can attribute earning capacity instead of using actual income when doing so serves the children’s interests.³ The judge looks at work history, skills, education, age, health, criminal record, the local job market, and whether employers are hiring. Bad faith is not required. The court only needs to find that the parent has both the ability and the opportunity to earn more.

Incarceration is the one firm limit. A jailed parent’s income is based on what they actually have access to, not what they earned before being locked up. Outside that exception, judges have wide latitude to plug a higher figure into the formula.

From Gross to Net Disposable Income

Family Code 4059 lists the deductions that convert gross income into the HN and TN the formula actually uses:⁴

Federal and state income taxes, based on the parent’s actual filing status, deductions, and credits, not just what the employer withholds.
FICA contributions (Social Security and Medicare), using the employee share or an equivalent amount for the self-employed.
Mandatory union dues and required retirement contributions, when they are conditions of employment.
Health insurance premiums for the parent and any children the parent is obligated to support, plus state disability insurance premiums.
Child or spousal support already being paid under another court order for children or a former spouse not involved in this case.
Necessary job-related expenses, at the court’s discretion.
Hardship deductions for qualifying situations such as extraordinary medical expenses or catastrophic losses.

Each parent’s deductions come off that parent’s own gross income. Both parents file an Income and Expense Declaration (form FL-150), attach pay stubs for the prior two months, and bring the most recent federal tax return to the hearing.⁵ Those documents are what the court uses to produce the net figures the formula needs.

How the Time-Share Percentage Is Measured

H% represents the fraction of total parenting time the higher earner has. Small changes to this number move the support amount more than most people expect. Courts calculate it from the hours set out in a custody and visitation order. If no formal schedule exists, the judge looks at the actual pattern of care over a representative period.

Overnight stays, weekday custody, and school breaks all count. School hours generally fall to whichever parent is responsible for the child during that block. A true 50/50 split produces an H% of 50. When different children have different schedules, the statute averages the percentages across all children covered by the order.¹ Inflating or shaving the numbers tends to get caught and tends to cost credibility with the judge.

Multipliers When More Than One Child Is Covered

The formula produces a figure for one child. When the order covers more, the court multiplies that base amount by a statutory factor that reflects shared household costs. Housing and utilities do not double just because a second child joins the first.¹

2 children: 1.6
3 children: 2.0
4 children: 2.3
5 children: 2.5
6 children: 2.625
7 children: 2.75
8 children: 2.813
9 children: 2.844
10 children: 2.86

The increments shrink quickly. Going from one child to two raises the total by 60 percent; going from nine to ten adds less than one percent. The total is generally divided among the children unless the court finds reason to allocate it differently.

The Low-Income Adjustment

SB 343 expanded the protection for low-earning payers. Under the current Family Code 4055(b)(7), when the paying parent’s net monthly disposable income falls below the gross monthly equivalent of full-time minimum wage (40 hours per week, 52 weeks per year), a rebuttable presumption kicks in that the parent qualifies for a reduction.¹ At California’s current $16.50 minimum wage, that threshold is roughly $2,860 per month in gross income, well above the approximately $2,100 threshold used before September 2024.

The reduction scales with the gap between the parent’s income and the threshold. The maximum reduction equals the standard support amount multiplied by a fraction whose numerator is the threshold minus the parent’s net income and whose denominator is the threshold. A parent with no income qualifies for the largest reduction; a parent just under the threshold gets a small one.

Because the presumption is rebuttable, the other parent can argue that applying the adjustment would be unfair, for instance when the paying parent has significant assets despite low reported income. If the adjusted amount still exceeds 50 percent of the paying parent’s net disposable income, the court can deviate further downward under Family Code 4057(b)(5).⁶

When a Judge Can Depart from the Guideline

The guideline number is presumed correct, but not untouchable. Family Code 4057 lists the grounds for ordering something different:⁶

Both parents stipulate in writing to a different amount.
The sale of the family home is deferred and the custodial parent gets a housing benefit that effectively reduces their costs.
A very high-income parent would be ordered to pay more than the children’s actual needs.
A parent is not contributing to the children in proportion to their custodial time.
Even after the low-income adjustment, support would exceed 50 percent of the paying parent’s net disposable income.
Special circumstances exist, including children with medical needs, different custody splits across children, equal time-sharing paired with very different housing costs, or a child with more than two legal parents.

To deviate, the court has to state in writing or on the record what the guideline would have produced, why the guideline amount would be unjust, and why the ordered amount serves the children’s best interests. Vague justifications get reversed on appeal, so judges tend to be careful here.

Putting the Formula to Work

Once you have each parent’s net monthly income, the higher earner’s time-share, and the number of children covered, the sequence is straightforward. Identify the TN tier and compute the base K fraction. Multiply by (1 + H%) or (2 − H%) depending on whether the higher earner has 50 percent or less of the parenting time or more. Multiply K by [HN − (H%)(TN)] to get the single-child support figure. Apply the multiplier if the order covers more than one child. Check whether the low-income adjustment applies, and check whether any of the Section 4057 grounds would justify deviation. The certified software does the arithmetic, but knowing what each step contributes lets you predict the order and spot an error in the inputs before it becomes an order you have to live with.

1
California Legislative Information. California Family Code 4055
2
California Courts. Guideline Support Calculators
3
California Legislative Information. California Family Code 4058
4
California Legislative Information. California Family Code FAM 4059
5
California Courts. Income and Expense Declaration (FL-150)
6
California Legislative Information. California Family Code 4057