Income Shares vs Percentage of Income
The two main support models can produce very different checks from the same paychecks. Here is the math and the fairness debate.
Income shares splits a combined obligation by income proportion, accounting for both parents; percentage of income takes a flat cut of the payer's earnings. Income shares is fairer when incomes are lopsided; percentage models are simpler and more predictable. Worked examples show the gap.
Same family, two formulas
Take parents earning $6,000 and $2,000 a month with one child. Under a simplified income-shares approach: combined income $8,000, basic obligation about $1,600 at 20 percent, split 75/25, so the higher earner's share is $1,200 and the lower earner's is $400. With the child living mostly with the lower earner, support flows from the higher earner's share, adjusted for time.
Under a flat 20 percent-of-income rule on the noncustodial parent: if the $6,000 earner is noncustodial, support is $1,200. Same answer here by coincidence of the numbers, but change the custodial parent's income and the models diverge fast.
Where they diverge
Give the custodial parent $10,000 a month and the noncustodial parent $4,000. Income shares sees $14,000 combined and a large obligation split 71/29, with the noncustodial parent's share around $800 of a $2,800 obligation. The percentage model still takes 20 percent of $4,000: $800. Similar again, because the percentage model ignores the custodial parent's wealth entirely.
Now flip it: custodial parent earns $2,000, noncustodial earns $12,000. Income shares: $14,000 combined, obligation about $2,800, noncustodial share 86 percent, roughly $2,400 before time credits. Percentage model: 20 percent of $12,000 is $2,400. The models agree more often than critics admit at middle incomes and diverge at extremes and with shared custody.
The fairness arguments
Income-shares supporters say children deserve a share of both parents' resources, and ignoring a wealthy custodial parent overcharges the poorer one. Percentage-model supporters say simplicity and predictability are worth it, and that the custodial parent already contributes through direct care and household spending.
Shared custody is where income shares shines: with 50/50 time, the transfer naturally shrinks toward the income difference, while flat-percentage rules need awkward bolt-on adjustments.
Which states chose which
The national trend has been toward income shares for decades; only a handful of states still use pure percentage models. Even percentage states have added income considerations, caps, and low-income adjustments over time.
No state is likely to switch models soon: changing formulas reshuffles thousands of existing orders, a political cost legislators avoid. Expect tweaks to tables, not model changes.
What it means for your case
You do not choose the model; your state does. Your leverage is in the inputs: verified income figures, documented overnights, and the add-ons for insurance and childcare.
Model the simplified version with the child support calculator to understand which inputs move your number most, then bring that understanding to your attorney or mediator.
Skip the arithmetic
See the income-shares math on your numbers with the free child support calculator.
Model comparison
What is the income-shares model?
The income-shares model adds both parents' monthly incomes, applies the state's table percentage for the child count, divides the resulting obligation between parents in proportion to their incomes, and then adjusts for parenting time, health insurance, and childcare costs.
What is the percentage-of-income model?
The percentage-of-income model sets support as a fixed share of the noncustodial parent's income, such as 20 percent for one child and 25 percent for two. It is simple and predictable but does not account for the custodial parent's earnings.
Which model do most states use?
Income shares dominates: the large majority of states use it. A small group, including Texas and New York with modifications, uses percentage of income. Delaware, Hawaii, and Montana use the Melson formula.