Child Support Income Shares Model Explained

If you live in one of 39 US states, your child support will be calculated using the Income Shares model. But understanding which model your state uses is only the beginning — the real challenge is knowing how the math works so you can verify whether the number you're given is actually correct.

How the Income Shares Model Works

The Income Shares model is built on a simple principle: a child should receive the same proportion of parental income they would have received if the family were still together. Both parents' incomes count, and both parents contribute proportionally.

Here's the calculation in four steps:

Step 1: Calculate each parent's adjusted gross monthly income (after allowable deductions).

Step 2: Add both incomes to get the combined adjusted income.

Step 3: Look up the basic support obligation on your state's schedule table. This table maps combined income and number of children to a dollar amount that represents the child's estimated needs.

Step 4: Divide the obligation proportionally. If Parent A earns $4,000 and Parent B earns $6,000, their combined income is $10,000. Parent A is responsible for 40% of the basic obligation, and Parent B covers 60%.

For example, if the state schedule says the basic obligation for one child at $10,000 combined monthly income is $1,200, Parent A owes $480 per month and Parent B owes $720. If Parent B is the custodial parent and already provides direct care, Parent A pays the $480 as child support.

The Percentage of Income Model

Nine states (including Texas, Illinois, and Mississippi) use the simpler Percentage of Income model, which looks only at the paying parent's income. A flat percentage is applied based on the number of children:

• One child: typically 17-20% of net income
• Two children: 25-28%
• Three children: 32-35%

Texas, for example, caps net resources at $9,200 per month (as of recent guidelines) and applies 20% for one child. A parent earning $7,000 net in Texas owes roughly $1,400 per month regardless of what the other parent earns.

The upside is simplicity. The downside is that it ignores the other parent's income entirely, which can produce unfair results when both parents have high or similar incomes.

The Melson Formula

Three states (Delaware, Hawaii, and Montana) use the Melson Formula — a more complex variation that first ensures each parent retains enough income for basic self-support, then allocates a percentage of remaining income to child support. It's designed to prevent the paying parent from falling below poverty while still meeting the child's needs.

Why Your State's Model Matters

The same family situation can produce very different support amounts under different models. Consider two parents with a combined $8,000 monthly income, split $5,000 and $3,000, with one child:

• Under Income Shares, the obligation might be $900, split 62.5% / 37.5% — the paying parent owes roughly $563
• Under Percentage of Income using 20% of the payor's net income ($5,000), the payment would be $1,000

That's a $437 monthly difference for the same family.

Beyond the Base Number

All three models calculate only the base obligation. Health insurance premiums for the child, work-related childcare costs, uninsured medical expenses, and sometimes extracurricular activities are added on top and typically split in proportion to each parent's income share.

Parenting time adjustments also layer on top — once a parent crosses the overnight threshold in their state, the base obligation decreases to account for the direct costs they incur during their parenting time.

The Child Support Estimation Worksheet Guide includes calculation worksheets for all three models, letting you estimate your obligation regardless of your state's approach and compare how the result would differ under alternative models.