How Child Support Is Calculated: Income Shares Model Explained

Important Disclaimer

Child support is determined by state courts under state-specific guidelines. Every US state has its own formula, income definitions, and deviation factors. The Income Shares Model described in this guide is a widely used framework that underpins the guidelines of most states, but the exact calculations — and the resulting amounts — differ by jurisdiction.

This calculator and guide provide a US-national approximation for educational and planning purposes only. Do not use this estimate in place of a state-specific calculation, legal counsel, or a court order.

Two Models, One Common Framework

US states use two primary child support models:

1. Income Shares Model (ISM): Used by the majority of US states. Based on the principle that children should receive the same proportion of each parent’s income they would have received if the parents lived together. Both parents’ incomes are combined, a support obligation is calculated based on that combined income, and each parent’s share is proportional to their income.

2. Percentage of Income Model: Used by a smaller number of states. The obligor (paying parent) pays a fixed percentage of their income regardless of the other parent’s income. This model is simpler but does not directly account for the custodial parent’s earnings.

This guide focuses on the Income Shares Model because it is more widely used and more nuanced.

The Income Shares Model: Step by Step

Step 1: Identify Both Parents’ Monthly Net Income

The ISM starts with the monthly net income of both parents. Net income generally means income after federal and state taxes, FICA, and mandatory deductions — the take-home amount available for support and living expenses.

Income sources typically included (varies by state):

• Wages, salary, tips, bonuses
• Self-employment income (net of business expenses)
• Investment income (dividends, interest, capital gains)
• Rental income
• Unemployment and disability benefits
• Social Security benefits

Some states may impute income to a parent who is voluntarily underemployed or unemployed.

Step 2: Combine Both Parents’ Net Incomes

Combined Monthly Net Income = Obligor Income + Obligee Income

Where:

• Obligor: The parent ordered to pay support (typically the non-custodial parent with fewer overnights)
• Obligee: The parent receiving support (typically the custodial parent or primary caregiver)

Step 3: Look Up the ISM Percentage

The combined income is applied to a schedule that assigns a support percentage based on the number of children. The percentages used in this calculator approximate the typical US ISM table:

Basic Monthly Support = Combined Income × ISM Percentage

This is the total support obligation as if both parents were together in one household providing for the children.

Step 4: Determine Each Parent’s Proportional Share

Each parent’s share of the total basic support obligation is proportional to their share of combined income:

Obligor Income Share = Obligor Income / Combined Income
Obligor Basic Support = Basic Monthly Support × Obligor Income Share

The obligee’s contribution is the remaining portion — they are presumed to be spending it directly on the children in the custodial household.

Step 5: Add Proportional Shares of Add-On Costs

Many state guidelines require both parents to share specific add-on expenses proportionally:

• Childcare costs: Work-related childcare (daycare, after-school care) necessary for the custodial parent to work or attend school
• Extraordinary medical expenses: Uninsured medical, dental, or mental health costs
• Health insurance premiums: The children’s share of health insurance

Total Add-On Costs = Monthly Childcare + Monthly Health Insurance + Other
Obligor Add-On Share = Total Add-On Costs × Obligor Income Share
Total Obligor Payment = Obligor Basic Support + Obligor Add-On Share

A Worked Example

Consider two parents:

• Obligor (paying parent): $6,000/month net income
• Obligee (custodial parent): $4,000/month net income
• 2 children
• Add-on costs: $300/month childcare + $150/month health insurance = $450/month total

Step 1 — Combined income: $6,000 + $4,000 = $10,000

Step 2 — ISM percentage (2 children): 25%

Step 3 — Basic monthly support: $10,000 × 25% = $2,500

Step 4 — Obligor income share: $6,000 ÷ $10,000 = 60%

Step 5 — Obligor basic support: $2,500 × 60% = $1,500

Step 6 — Add-on costs: $450/month total; obligor’s 60% share = $270

Total Obligor Payment: $1,500 + $270 = $1,770/month

Parenting Time Adjustments

One of the most significant factors not captured in this basic calculation is parenting time. Many states reduce the support obligation when the non-custodial parent has significant overnights with the children — often called the “shared custody” or “shared parenting” adjustment.

The logic: when a child spends substantial time with both parents, both parents are directly incurring costs during those periods. A strict ISM without adjustment would require the non-custodial parent to both pay child support and bear all the direct costs during their time.

The threshold for a parenting time adjustment and the formula used to calculate it varies by state. Some states use a threshold of 90 overnights per year; others start at 40%; others have a continuous sliding scale.

This calculator does not include a parenting time adjustment. If you have significant parenting time, your actual state-calculated obligation may be lower than this estimate.

What the Obligee’s “Contribution” Looks Like

Under the ISM, the obligee (custodial parent) is presumed to be spending their proportional share — 40% in the example above ($2,500 × 40% = $1,000) — directly on the children in the custodial household through day-to-day expenses: food, clothing, housing costs allocated to the children’s space, school supplies, and general care.

The obligor’s payment ($1,770 in the example) is the cash transfer that funds the obligor’s proportional share of costs that are actually spent through the custodial household.

Modifications and Deviations

Courts can deviate from the guideline amount in both directions when substantial evidence supports it. Common grounds for deviation:

• High-income families: At very high combined incomes, the percentage model may produce support far in excess of the children’s actual needs. Courts may set a ceiling.
• Special needs: A child with disabilities or extraordinary medical needs may require higher support.
• Other children: A parent supporting other children from another relationship may have the obligation adjusted downward.
• Shared physical custody: As noted, significant parenting time reduces the obligor’s share.
• Travel costs: When parents live far apart, travel costs for parenting time can be factored in.
• Visitation interference: Courts rarely, but occasionally, consider patterns of access interference.

Support orders can be modified when circumstances change substantially — a job loss, a large income change, a change in custody arrangements, or a child becoming emancipated.

Child Support and Taxes

Under current federal tax law (Tax Cuts and Jobs Act of 2017, which was made permanent for child support through related legislation):

• Child support received is not taxable income for the recipient parent
• Child support paid is not tax-deductible for the paying parent

This is different from alimony (for divorces finalized before 2019, alimony was deductible/includable; for divorces after 2018, it is neither).

The dependency exemption and child tax credit are separate determinations. They typically go to the custodial parent by default but can be allocated by agreement or court order.

Using This Calculator for Planning

While every state’s formula is different, this calculator gives a useful planning estimate for:

• Understanding the ballpark: Even imprecise estimates help set expectations before consulting an attorney.
• Exploring how-ifs: What happens to the estimate if your income changes? If you get joint custody? If add-on costs increase?
• Settlement negotiations: Parties negotiating support agreements often start with the guideline calculation, then negotiate adjustments. Knowing the approximate guideline figure anchors the conversation.
• Budgeting: Either parent can use the estimate to start budgeting for the monthly obligation.

Summary

The Income Shares Model calculates child support by combining both parents’ net incomes, applying a percentage by number of children (17% for 1 child through 33% for 6+), assigning each parent a proportional share, and adding proportional add-on costs. For the worked example — $6,000 obligor and $4,000 obligee monthly net income, 2 children, $450/month add-ons — the estimated monthly support is $1,770.

This is a US-national approximation. Use the child support calculator to estimate based on your specific situation, and consult a family law attorney for state-specific guidance.

This guide is for educational and estimation purposes only. Child support is a legal determination made under state-specific guidelines by a court. Nothing here constitutes legal advice.