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. Per 45 CFR §302.56, states must adopt a guideline, review it at least every four years, and apply it as a rebuttable presumption — but the choice of model is left to each state. The federal cross-state reference for these models is maintained by the HHS Office of Child Support Services (OCSS). 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.

In my experience modeling these guidelines for individuals trying to plan ahead, the model effect is the single thing people most consistently miss — they fixate on their income and the number of kids, and never realize that the formula their state happens to use is doing more work than either input. I've found that the same $8K-obligor / $4K-custodial / 2-children fact pattern produces materially different obligations depending only on which model governs:

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

• Income Shares (typical Midwest state): ~$1,650–1,800/month (obligor pays ~66.7% of a combined-income basic obligation, plus apportioned add-ons)
• Percentage of Obligor Income (Wisconsin): ~$2,000/month (25% of the obligor's $8K, custodial income ignored)
• Melson Formula (Delaware): ~$1,750–1,900/month (SSR floor + primary need + a SOLA slice of discretionary income)

That spread — roughly $1,650 to $2,000 on the identical family — is about a 30% swing driven purely by which model applies. I've seen people assume a number from a friend in another state must be wrong; usually it's just a different model. Differences this large reflect deep policy choices, not noise. If you're weighing 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. None of these figures is a prediction of what a court will order; they are estimates of guideline math, and the actual order depends on your facts, your state, and your county. Consult a licensed family-law attorney.

How it’s calculated

The Child Support Estimator implements the three models as explicit formulas, then layers the add-on apportionment and shared-custody adjustment on top. Here is the math it runs:

MODEL 1 — Income Shares
  CAGI            = obligor income + custodial income
  Base Obligation = state guideline schedule(CAGI, # children)
  Obligor share % = obligor income ÷ CAGI
  Obligor support = Base Obligation × Obligor share %

MODEL 2 — Percentage of Obligor Income
  Obligor support = obligor income × state statutory %(# children)
                    (custodial income is NOT used)

MODEL 3 — Melson Formula (three steps)
  1. Self-Support Reserve (SSR): reserve obligor income below
     the state's self-support floor (tied to poverty guideline)
  2. Primary Support Need (PSN): state minimum per child
  3. Standard of Living Allowance (SOLA): a % of the income
     remaining after SSR + PSN are funded
  Obligor support = obligor share of (PSN + SOLA), applied
                    only to income above the SSR floor

ADD-ON APPORTIONMENT (Income Shares / Melson)
  Each parent's add-on share = (health insurance + childcare)
                               × that parent's income share %
  (Percentage-of-Obligor states usually add these on top,
   un-apportioned)

SHARED-CUSTODY ADJUSTMENT
  If non-custodial overnights ≥ state threshold
     (commonly 92, 110, or 128/yr — state-specific):
     reduce / cross-credit the obligation per the state formula
  Else: standard guideline math applies

Assumptions: state guideline schedules and statutory percentages vary by state and county — the figures shown in the worked examples are illustrative, not your state's actual numbers. Shared-custody overnight thresholds (commonly 92, 110, or 128 per year) and the adjustment formulas above them are state-specific. The estimator does not model deviations from the guideline (judicial discretion factors), prior support orders, multiple-family or stepfamily situations, or interstate enforcement under UIFSA. Figures are estimates of guideline math, not legal advice, and not a prediction of what a court will order. Read the child support & custody methodology for the per-state formula derivations, or the full methodology for how every page on this site is built and reviewed. Try the live Child Support Estimator to run these formulas against your own inputs, or view the open-source math on GitHub at packages/calc/src/child-support.ts.

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

Related calculators on this site

Child support is one piece of a larger legal-cost picture. If you're modeling the full financial impact of a legal proceeding, these companion estimators run the same show-your-work math against published primary sources:

• Child Support Estimator — applies your state's guideline model to your income, custody, and add-on inputs.
• Lost Wages Calculator — present-value of lost earnings using BLS Work-Life Tables (personal-injury context).
• Contingency Fee Calculator — attorney-fee math under ABA Model Rule 1.5 and state fee caps.