Child Support Income Shares Model: How 41 States Calculate Obligations in 2026

Every state uses one of three child support calculation models, and 41 of them use the Income Shares model. The idea is straightforward: a child should receive the same proportion of parental income they would have gotten if the family lived together. The court combines both parents' incomes, looks up a basic obligation on the state's schedule, and splits it based on each parent's share of combined income. But the mechanics involve gross-to-net income conversions, standardized tax computations, parenting time adjustments, and add-on expenses for health insurance and childcare. Iowa updated its schedule on January 1, 2026, increasing obligations by an average of 7.6% for one child. Illinois updated its schedule and conversion tables effective March 20, 2026. Use our Child Support Guideline Calculator to estimate obligations under your state's Income Shares model.

The Three Child Support Models

The National Conference of State Legislatures tracks which model each state uses. Washington, D.C. runs a hybrid that starts as a percentage of income and then adjusts based on the other parent's income.

How the Income Shares Model Works

The calculation follows five steps:

Step 1: Calculate each parent's adjusted net monthly income. Start with gross monthly income from all sources: salaries, wages, commissions, overtime, bonuses, self-employment income, dividends, interest, Social Security benefits, workers' compensation, and unemployment benefits. Subtract mandatory deductions: federal and state income tax (using the guidelines' standardized tax computation), Social Security tax (FICA at 7.65% for employees, 15.3% for self-employed), mandatory retirement contributions, union dues, health insurance premiums for the parent only, and prior child support or spousal support obligations for other families.

Step 2: Combine the adjusted net incomes. Add both parents' adjusted net monthly incomes to get the combined adjusted net income.

Step 3: Look up the basic child support obligation (BCSO). Use the state's Schedule of Basic Support Obligations, which provides a dollar amount based on combined income and number of children. The schedule covers combined incomes from $100 to $30,000+ per month and provides amounts for 1 through 6 or more children.

Step 4: Split the BCSO by income share. Each parent's share of the BCSO equals their proportion of combined income. If Parent A earns $4,500 and Parent B earns $3,000, combined income is $7,500. Parent A's share is 60%. Parent B's share is 40%.

Step 5: Adjust for parenting time and add-on expenses. The parent with fewer overnights typically pays their share to the other parent. Parenting time credits reduce the obligation for parents with significant overnight visitation. Health insurance premiums for the children and work-related childcare costs are added to the BCSO and split by the same income shares.

Step-by-Step: Income Shares Calculation

Consider two parents in an Income Shares state. Parent A (custodial parent) earns $4,200 gross per month. Parent B (non-custodial parent) earns $5,800 gross per month. They have two children. Parent B has the children 104 overnights per year.

Step 1: Adjusted net income. Using the state's standardized tax computation:

• Parent A: $4,200 gross minus $640 federal tax, $210 state tax, $321 FICA, $180 health insurance = $2,849 adjusted net
• Parent B: $5,800 gross minus $950 federal tax, $310 state tax, $444 FICA, $180 health insurance = $3,916 adjusted net

Step 2: Combined income. $2,849 + $3,916 = $6,765

Step 3: Schedule lookup. For combined adjusted net income of $6,765 and two children, the state schedule shows a BCSO of approximately $1,650 per month. (Actual amounts vary by state. Iowa's 2026 schedule shows an average 10.5% increase for two children compared to the prior cycle.)

Step 4: Income shares.

• Parent A's share: $2,849 / $6,765 = 42.1%
• Parent B's share: $3,916 / $6,765 = 57.9%
• Parent A's obligation: $1,650 x 0.421 = $694.65
• Parent B's obligation: $1,650 x 0.579 = $955.35

Step 5: Parenting time adjustment. Parent B has 104 overnights, which is approximately 28.5% of the year. The state applies a parenting time credit that reduces Parent B's obligation. If the credit is 25%: $955.35 x 0.75 = $716.51.

Add-on expenses. Children's health insurance costs $180/month (paid by Parent B). Work-related childcare costs $400/month (paid by Parent A). Total add-ons: $580, split by income shares:

• Parent A's share: $580 x 0.421 = $244.18
• Parent B's share: $580 x 0.579 = $335.82

Final obligation. Parent B pays Parent A: $716.51 (BCSO after parenting time credit) + $335.82 (add-on share) - $180 (health insurance already paid by Parent B) = $872.33 per month.

2026 Schedule Updates

Several states updated their child support schedules in 2026:

• Iowa: Effective January 1, 2026, the Iowa Supreme Court updated the Schedule of Basic Support Obligations following a September 29, 2025 court order. The update reflects a 21% increase in consumer prices since the prior cycle, resulting in average obligation increases of 7.6% for one child, 10.5% for two children, and 11.6% for three children.
• Illinois: Updated both the Gross to Net Income Conversion Table and the Income Shares Schedule of Basic Child Support Obligations effective March 20, 2026. Illinois is also conducting its quadrennial review in 2026, as required by federal law.
• Washington: Updated its economic table effective June 2026, with revised allocation standards under RCW 26.19.080.

States are required by federal law to review their child support guidelines at least once every four years to ensure they remain appropriate for the children and families they serve. Many states tie schedule updates to changes in the Consumer Price Index.

Common Mistakes in Income Shares Calculations

Using gross income instead of adjusted net income. The Income Shares model uses adjusted net income, not gross. The standardized tax computation in most states produces a net income figure that differs from what a parent actually takes home because it uses standardized tax tables rather than the parent's actual tax return. Always use the state's official conversion table.

Not including all income sources. Gross income includes overtime, bonuses, commissions, second jobs, dividend and interest income, Social Security benefits, workers' compensation, unemployment benefits, and self-employment income. Leaving out any of these sources understates the combined income and produces an incorrect BCSO.

Misapplying the parenting time credit. The parenting time credit applies only to the BCSO, not to add-on expenses. Some states require a minimum number of overnights before the credit applies. In Iowa, the extraordinary visitation threshold is 127 overnights per year. Check your state's specific threshold.

Not adjusting for prior support obligations. If a parent has existing child support or spousal support obligations from other relationships, these are deducted from gross income before calculating adjusted net income. Failing to deduct prior obligations overstates the parent's available income and inflates the obligation.

Forgetting the self-employment tax difference. Self-employed parents pay both halves of FICA (15.3%) instead of the employee rate (7.65%). The standardized tax computation accounts for this, but if you are doing a manual calculation, use the correct rate.

Related Tools on ProfessionCalculators.com

• Child Support Guideline Calculator to estimate child support under your state's model
• SNAP Benefits Eligibility Estimator to check whether child support payments affect SNAP eligibility
• Benefits Cliff Analyzer to see how child support payments interact with benefit eligibility

FAQ

What is the Income Shares child support model? The Income Shares model combines both parents' adjusted net incomes, looks up a basic child support obligation on the state's schedule based on combined income and number of children, and splits the obligation in proportion to each parent's share of combined income. It is used by 41 states.

How is adjusted net income calculated for child support? Start with gross monthly income from all sources. Subtract federal income tax, state income tax, FICA (7.65% for employees, 15.3% for self-employed), mandatory retirement contributions, union dues, parent-only health insurance premiums, and prior support obligations. The result is adjusted net income.

What is the basic child support obligation (BCSO)? The BCSO is the dollar amount from the state's schedule of basic support obligations, based on the parents' combined adjusted net income and the number of children. It covers average child-rearing expenses including housing, food, transportation, clothing, and entertainment.

How does parenting time affect child support? Most Income Shares states apply a parenting time credit that reduces the paying parent's obligation based on the number of overnights they have with the child. The credit typically applies only to the BCSO, not to add-on expenses like health insurance and childcare. Thresholds vary by state.

How often do states update their child support schedules? Federal law requires states to review their child support guidelines at least once every four years. Many states update their schedules more frequently to reflect changes in the Consumer Price Index. Iowa, Illinois, and Washington all updated their schedules in 2026.

Conclusion

The Income Shares model is the most widely used child support calculation framework in the United States, and its accuracy depends on getting each step right. Start with gross income from all sources, apply the state's standardized tax computation to get adjusted net income, combine both parents' incomes, look up the BCSO on the current schedule, split by income share, and adjust for parenting time and add-on expenses. States update their schedules periodically to reflect inflation, so always use the current year's schedule. For 2026, several states including Iowa and Illinois have updated their schedules with increases ranging from 7.6% to 11.6% depending on the number of children.