Child Support Formulas Explained

Every US state uses one of three models to calculate child support. Which model your state uses determines whether only the non-custodial parent's income matters, or both parents' incomes are considered — and that single fact can mean thousands of dollars of difference per year.

Three Formula Models Compared

The three guideline models in use across US states differ in input variables, computation steps, and how they treat low-income obligors:

Feature	Income Shares	Percentage of Income	Melson
States using	42	6	3
Both parents' incomes	Yes — combined	Obligor only	Yes — combined
Self-support reserve	Optional sub-table	Rare	Mandatory first step
Schedule lookup table	Yes	No — flat rate	Yes — for primary need
Standard of living adjustment	Implicit in schedule	No	Explicit SOLA layer

Worked Example: $4,500/Month Gross, One Child

Take a non-custodial parent earning $4,500 gross monthly with no parenting time credit. Under Income Shares with the other parent earning $2,500/month, combined income is $7,000/month, the schedule shows $1,400 total obligation, and the obligor's 64.3% income share yields approximately $900/month. Under Texas Percentage of Income at 20% of obligor income, the order computes directly to $900/month. Under Delaware Melson with a $1,200 self-support reserve, the obligor's available income is $3,300, primary need at 22% yields $726, and the SOLA layer adds approximately $145, totaling roughly $871/month.

"Although every state must have child support guidelines, federal law leaves the formula choice to each state legislature." — Family Support Act of 1988, codified at 42 USC §667

Model 1: Income Shares (41 states + DC)

The Income Shares model is used by the majority of states. The core concept: children should receive the same proportion of parental income they would have received if their parents were still together.

How it works:

1. Combine both parents' incomes. Add the gross monthly incomes of both parents.
2. Look up the basic obligation. Use your state's income schedule to find the total obligation for your combined income and number of children.
3. Split by income ratio. Divide the total obligation proportionally — if the non-custodial parent earns 60% of combined income, they pay 60% of the obligation.
4. Add adjustments. Subtract parenting time credits, add work-related childcare, add health insurance costs.

Example (California, 1 child): Non-custodial earns $5,000/mo, custodial earns $3,000/mo. Combined = $8,000/mo. Approximate obligation from schedule: ~$1,520. Non-custodial's share = 62.5% = ~$950/mo base (before parenting time and other adjustments).

Model 2: Percentage of Income (6 states)

The Percentage of Income model is simpler: apply a fixed percentage to the non-custodial parent's income. The custodial parent's income is generally irrelevant.

States using this model: Wisconsin, Illinois, North Dakota, Mississippi, Nevada, and Texas.

Example (Texas):

• 1 child = 20% of net monthly resources
• 2 children = 25%
• 3 children = 30%
• Income cap: $9,200/mo net resources

A parent earning $5,000/mo net pays exactly $1,000/mo for one child — regardless of what the other parent earns.

Model 3: Melson Formula (3 states)

The Melson Formula is the most complex model, used only by Delaware, Hawaii, and Montana. Named after Delaware Judge Elwood Melson who developed it in the 1970s, it prioritizes ensuring both parents can meet their own basic needs before calculating child support.

Three steps:

1. Self-Support Reserve (SSR). First, each parent is entitled to keep enough income to meet their own basic needs (typically $850–$1,100/mo). Only income above the SSR is available for child support.
2. Primary Support Need (PSN). A fixed dollar amount per child is set as the minimum required. Both parents contribute proportionally from their income above the SSR.
3. Standard of Living Adjustment (SOLA). If any income remains after meeting the PSN, the non-custodial parent must contribute a percentage (typically 15%) to improve the child's standard of living.

Which Model Produces Higher Support?

It depends on the income split. When the custodial parent earns significantly less than the non-custodial parent:

• Income Shares tends to produce higher support (the high-earning obligor pays a larger share)
• Percentage is predictable — always the same % of obligor income
• Melson can be lower when the obligor earns near the self-support reserve, but higher for higher earners due to SOLA

What All Models Have in Common

Regardless of model, all states allow courts to deviate from guideline amounts under certain circumstances, and most include add-ons for:

• Work-related childcare expenses
• Health insurance premiums
• Extraordinary medical expenses
• Educational expenses (in some states)