What is the income shares model in California?

The income shares model assumes that both parents should contribute to their child's support in proportion to their incomes. California combines both parents' net disposable incomes, determines the total support obligation from a statewide table, then allocates that obligation between the parents based on their relative incomes and custody time.

What income counts for child support calculations?

California Family Code § 4058 defines income broadly to include wages, salaries, bonuses, commissions, self-employment income, rental income, investment income, unemployment benefits, workers' compensation, disability, retirement income, and other sources. Some forms of public assistance (like SSI) are excluded.

How does the K-factor affect my child support?

The K-factor adjusts the support obligation based on how much time each parent has the children. It ranges from 1.0 (when the noncustodial parent has 0% timeshare) to 2.0 (when timeshare approaches 50/50). Higher K-factors reduce the net transfer payment because both parents are spending significant time and resources directly on the children.

What are the mandatory add-on costs?

California requires parents to share certain add-on costs proportionally: (1) health insurance premiums for the children, (2) work-related childcare costs, and (3) uninsured medical expenses. These are added to the basic guideline amount and split according to each parent's percentage of combined income.

Can child support deviate from the guideline formula?

Yes, but only under specific circumstances outlined in Family Code § 4057. Courts may deviate if following the guideline would be unjust or inappropriate, considering factors like extreme hardship, extraordinarily high income, additional children from other relationships, or special needs. Any deviation must include written findings explaining the reasons.

How did SB 343 change the formula?

SB 343 (effective January 1, 2024) updated the economic data tables used in the guideline calculations to reflect current costs of raising children. It also refined K-factor calculations and hardship considerations. The basic formula structure remains the same, but the underlying cost data is now more recent.

What is the self-support reserve?

The self-support reserve (FC § 4055(b)(7)) is a minimum income amount protected for the paying parent to meet their own basic needs. If calculating guideline support would reduce the payor's income below this reserve, the court may set support at a lower amount to ensure the parent can maintain a minimum standard of living.

Do I need special software to calculate child support?

While the formula is publicly available, most attorneys and courts use specialized software like DissoMaster or XSpouse because the calculations involve complex interactions between income, deductions, K-factors, and add-ons. Our online calculator implements the same formula for free, providing estimates that match professional software results.

California Family Code § 4055

California Child Support Formula: Complete 2025 Guide

Understand how California calculates child support using the income shares model under Family Code § 4055, including K-factor time-share adjustments, mandatory add-ons, and the SB 343 updates effective January 2024.

Important Note

This guide explains the guideline formula. Actual court orders may deviate based on specific circumstances. Always consult with a family law attorney for personalized advice.

Calculate Support See Formula Details

Key Formula Facts

Formula Type

Standard

Income Shares

Considers both parents' incomes

California Family Code § 4055

Income Definition

Gross Income

Before tax deductions

FC § 4058

Calculation Method

Mandatory

Statewide Uniform

Same formula for all counties

FC § 4053

K-Factor Range

1.0 to 2.0

Based on parenting time split

FC § 4055(b)(1)(B)

Effective Date

Updated

January 1, 2024

SB 343 updates applied

SB 343 (2023)

Hardship Reserve

Self-Support Reserve

Minimum income protected

FC § 4055(b)(7)

Formula Components

Combined Net Disposable Income

Sum of both parents' net monthly income after allowed deductions

Parent A Net + Parent B Net

Example: $8,000 + $5,000 = $13,000

Proportion of Income

Each parent's share of combined income

(Parent Income ÷ Combined Income) × 100

Example: Parent A: ($8,000 ÷ $13,000) × 100 = 61.5%

Basic Obligation

Total support from statewide guideline table

Table Lookup (Combined Income × # Children)

Example: $13,000 income, 2 children → $2,340/month

K-Factor Adjustment

Modifies obligation based on parenting time

H × (1 + K)

Example: K=1.5 for 30% timeshare with noncustodial parent

Add-On Costs

Health insurance, childcare, uninsured medical

Proportional Share × Add-On Cost

Example: 61.5% × $300 health premium = $185/month

Final Obligation

Net support payment after all adjustments

Basic + Add-Ons - Deductions

Example: $1,440 basic + $185 add-ons = $1,625/month

Step-by-Step Calculation Process

Step 1

Calculate Each Parent's Net Disposable Income

Start with gross income, subtract allowed deductions (taxes, mandatory retirement, health insurance, union dues, child support for other children)

Example: Gross $10,000 - $2,500 taxes - $500 insurance = $7,000 net

Step 2

Determine Combined Net Income

Add both parents' net disposable incomes together

Example: Parent A $7,000 + Parent B $4,000 = $11,000 combined

Step 3

Look Up Basic Support Obligation

Use statewide table based on combined income and number of children

Example: $11,000 combined, 2 children → $1,980/month base obligation

Step 4

Calculate Each Parent's Proportional Share

Divide each parent's net income by combined net income

Example: Parent A: $7,000 ÷ $11,000 = 63.6%; Parent B: 36.4%

Step 5

Apply K-Factor for Timeshare

Adjust obligation based on percentage of time each parent has custody

Example: Parent B has child 20% of time → K-factor increases obligation

Step 6

Add Mandatory Add-Ons

Include health insurance premiums, childcare costs, uninsured medical expenses (proportional to income share)

Example: Parent A pays 63.6% of $400 health premium = $254/month

Step 7

Calculate Net Payment

Higher-earning parent pays lower-earning parent their share minus their own custody credit

Example: Parent A owes $1,258/month (basic) + $254 (add-ons) = $1,512 total

Frequently Asked Questions

Related Resources

California Child Support Calculator

Calculate your estimated support payment using our free online tool

Deviation Factors Guide

Learn when and how courts may deviate from guideline support

SB 343 Policy Updates

Understand the 2024 changes to California child support law

Child Support Modification

How to request changes to an existing support order

Legal References & Review

Content Review

Last Reviewed: January 18, 2025
Next Review: July 18, 2025
Reviewed By: Family Law Research Team

Content reviewed for accuracy and compliance with California Family Law as of January 2025

Effective Law

Family Code § 4050-4076, SB 343 (2023)

Content based on California law as of January 2025. Always verify current requirements with local court.

Primary Legal Sources:

California Family Code § 4055 - Child Support Guideline Formula(statute)
California Family Code § 4058 - Annual Gross Income(statute)
SB 343 (2023) - Child Support Guideline Updates(legislative)
California Family Code § 4057 - Rebuttal of Guideline(statute)

Internal Research

This content incorporates research from our comprehensive California child support analysis. For additional context, see our 2025 Policy Update and California Calculator.