Income Shares vs Percentage of Obligor Income vs Melson: the three child-support models, decoded

Why three models exist

Federal regulation (45 CFR §302.56) requires every state to publish a numeric guideline for child support but doesn't prescribe the formula. States chose between three policy theories, each with a different answer to the same question: how much of household income should children receive?

• Income Shares theory: children should receive the same proportion of parental income they would have received if the family remained intact. Both parents' incomes matter.
• Percentage of Obligor Income theory: non-custodial parent's income should pay a flat percentage scaled by number of children. Custodial parent's income doesn't enter the formula. Simple, low-dispute.
• Melson theory: hybrid — Income Shares logic, plus a Self-Support Reserve floor for the obligor, plus a Standard of Living Allowance (SOLA) that gives the child a share of higher household standards.

Model 1: Income Shares (38 states + DC)

Most common; most widely used. Adopted by Arizona, California, Colorado, Connecticut, Florida, Georgia, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Missouri, Nebraska, New Hampshire, New Jersey, New Mexico, New York, North Carolina, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Utah, Vermont, Virginia, Washington, West Virginia, Wyoming, and DC.

Example: Obligor earns $8,000/month, custodial parent
$4,000/month, 2 children.

Step 1: Combined Adjusted Gross Income (CAGI)
    $8,000 + $4,000 = $12,000

Step 2: Basic Child Support Obligation (from state table)
    State guideline schedule says $12,000 CAGI with
    2 children = $1,920/month
    (this number is state-specific — see your state's
    schedule)

Step 3: Obligor's proportional share
    Obligor share % = $8,000 / $12,000 = 66.7%
    Obligor's BCSO = $1,920 × 66.7% = $1,280/month

Step 4: Add-ons (apportioned by income share)
    + Health insurance for kids $200/mo × 66.7% = $133
    + Childcare $400/mo × 66.7% = $267

= $1,680/month obligation

Source: state-specific statute. California uses Cal. Fam. Code §4055; New York uses NY DRL §240(1-b); Florida uses Fla. Stat. §61.30; Texas uses Tex. Fam. Code §154 (variant with statutory cap). Each state publishes its own guideline schedule — the $1,920 figure above is illustrative and varies by state and family size.

Model 2: Percentage of Obligor Income (6 states)

Used by Alaska, Arkansas, Mississippi, Nevada, North Dakota, Wisconsin. Texas uses a percentage-cap variant.

Same example: Obligor earns $8,000/month, 2 children.
(Custodial parent's income is NOT in the formula.)

Statutory percentage from state code (varies by state):
    Wisconsin 2 children: 25%
    Texas      2 children: 20%
    Mississippi 2 children: 20%
    Nevada     2 children: 25%

Calculation (using Wisconsin rate):
    $8,000 × 25% = $2,000/month

Add-ons (childcare, health insurance) typically additional
to the percentage, NOT apportioned.

Simpler math, fewer disputes. Critics argue it ignores the custodial parent's income — a wealthy custodial parent receives the same percentage from a struggling obligor as a struggling custodial parent would. Defenders argue this is exactly the point: support is for the children, calculated against the non-custodial parent's capacity to pay, independent of the custodial household's wealth.

The same $8K/month obligor produces a $1,680/month obligation under Wisconsin's Income Shares (if Wisconsin used Income Shares, hypothetically) but a $2,000/month obligation under Wisconsin's actual Percentage of Obligor Income formula — a 20% difference for the identical fact pattern. That's the model effect.

Model 3: Melson Formula (3 states)

Delaware, Hawaii, Montana. Most progressive across income ranges; most complex math.

Three-step Melson calculation:

Step 1: Self-Support Reserve (SSR) for obligor
    State sets a minimum living standard threshold —
    typically tied to federal poverty guideline.
    The obligor's income BELOW the SSR is reserved
    for their basic self-support; no support paid
    from below-SSR income.

Step 2: Primary Support Need (PSN) per child
    State schedule sets minimum support need:
    typically $300–500 per child per month.

Step 3: Standard of Living Allowance (SOLA)
    After SSR + PSN are funded, additional discretionary
    support based on remaining household income.

The SOLA component is what makes Melson "progressive" —
it scales the child's support level up as the family's
combined disposable income increases.

Same $8K/month obligor with the custodial parent earning $4K and 2 children would produce a moderately different number under Melson than under Income Shares — typically lower at very low incomes (because the SSR protects basic obligor self-support) and slightly higher at very high incomes (because SOLA gives the child a piece of the high-end discretionary income). For middle-income families, Melson typically lands within 5-10% of Income Shares.

Why the model effect is bigger than the income effect

The most interesting empirical observation: across the same family fact pattern (same incomes, same number of kids, same custody split), the choice of model can shift the resulting obligation by 15–30%. That's often a bigger effect than a $1,000/month income change within the same state.

Concrete example: $8K obligor + $4K custodial + 2 children + standard custody (every other weekend):

• Income Shares (typical Midwest state): ~$1,650–1,800/month
• Percentage of Obligor Income (Wisconsin): ~$2,000/month
• Melson Formula (Delaware): ~$1,750–1,900/month

Differences this large reflect deep policy choices, not noise. If you're evaluating a job offer that would change your state of residence, the child-support math is a legitimate financial input — but it's rarely captured in the spreadsheet.

What this calculator does + doesn't do

The Child Support Estimator on this site:

• Detects which model your state uses
• Applies the correct formula with your income inputs
• Surfaces add-ons (health insurance, childcare) apportioned by your state's rule
• Applies shared-custody adjustment if applicable
• Cites the primary statute so you can verify each step

What it does NOT do:

• Model deviations from the guideline (judicial discretion factors)
• Account for prior support orders, multiple-family situations, or stepfamily adjustments
• Apply interstate enforcement rules under UIFSA
• Replace a family-law attorney for filing, modification, or enforcement