The model most states use
Most states — 41, by one published count — use the income-shares model. It starts from a simple idea: the child should receive the same proportion of parental income they would have had if the parents still lived together. Both parents' gross incomes are combined, and the state's schedule gives a basic support obligation for that combined income and that number of children. Each parent's share of the obligation is their income over the combined total — and that single ratio does most of the work, because it also splits the add-ons, including the health insurance premium.
A handful of states use the percentage-of-income model instead, which applies a fixed percentage to the payer's income alone — Wisconsin's 25% for one child is the cleanest published example, and Texas and Alaska use percentage models too. In those states the premium is handled as an add-on to the base obligation or deducted from the paying parent's income before the percentage is applied, depending on the state. Either way, the premium is inside the calculation, not outside it.
Federal law requires every state to publish a numeric guideline, to apply it as a rebuttable presumption in every case, and to review it at least once every four years against economic data on what families actually spend on children (45 CFR 302.56). The guideline is a presumption, not a sentence: a court can depart from it, but it must give reasons. The schedule figures themselves are set state by state and change with each review, so no dollar figure in this guide is a substitute for your state's schedule.
How the premium is counted
The cost counted is the incremental cost of adding the child to the plan — the difference between the premium with the child and without the child — not the whole family premium. If a parent already has family cover for a second family, adding this child may cost nothing extra, and in that case no adjustment is made: there is no premium to split. The parent must prove the increase with evidence that identifies the child's addition as the source of the cost, and the cost must be reasonable.
Reasonable has a federal definition. Under 45 CFR 303.31, health insurance is considered reasonable in cost when the premium does not exceed 5% of the responsible parent's gross income, though states can set a different threshold — in practice most state thresholds fall between 5% and 9% of gross income. If no parent has access to insurance at a reasonable cost, the order can instead require cash medical support — a payment toward the child's medical costs, commonly capped at 5% of the obligor's gross income.
Once the plan is selected, the carrying parent typically pays the premium upfront through payroll deductions, and the court adjusts the base support amount to reflect the contribution. If one parent earns 60% of the combined income and pays the full premium, the other parent effectively owes 40% of that premium, and the carrying parent's support obligation drops by that amount. The adjustment is rarely dollar-for-dollar in the way parents expect — it is a credit for the other parent's share, not a refund of the whole premium.
A worked example
Take the example used in published guidance: one parent earns $6,000 a month and the other earns $4,000, so the combined income is $10,000 and the shares are 60% and 40%. Adding the child to a health plan costs $300 a month. The first parent's share is $180 and the second's is $120. If the first parent carries the cover and pays the full $300, the second parent owes $120 of it, and the first parent's support obligation is reduced by $120 — not by $300.
The same ratio is applied to unreimbursed medical expenses — copays, deductibles, orthodontia, therapy and any treatment the plan does not cover. Courts typically split those costs between the parents on the same income-based ratio, or on a straight 50/50 division, depending on the state, and the split runs on top of the regular support payment. A $4,000 orthodontia bill with a 60/40 split means $2,400 from one parent and $1,600 from the other, regardless of the monthly support amount. Some states require the receiving parent to pay the first $250 of unreimbursed expenses before the split begins — check your state's rule, because that detail changes the arithmetic.
Who has to provide the cover
Every child support order enforced through a state's child support agency must include a provision for medical support — that is a federal requirement, not an option. The order usually names the parent who has access to coverage at a reasonable cost, and requires that parent to enrol the child. If the carrying parent's employer offers family cover, the court can send a National Medical Support Notice to the employer, which requires the employer to enrol the child in the parent's plan.
If neither parent has access to affordable employer cover, the order can require the child to be enrolled in Medicaid or a state children's health programme where eligible, or can order cash medical support instead. The parent who carries the cover must keep it in force and must tell the other parent — and the child support agency — if it lapses, changes or becomes unavailable. Losing cover without notice is the most common way a medical support order is breached, and it is usually treated as a failure to comply with the order, not as a change of circumstances.
The site's guide to employer health insurance explains how group cover works, and the health insurance comparison page sets out the products — but for a support order, the plan document and the state guideline are the authorities, and this page is general information only, not legal advice. If your case is contested, the guideline schedule and a family lawyer are the next step, not a general guide.
Worked example
Consider two ILLUSTRATIVE parents. The first earns $6,000 a month, carries the child's health cover at an incremental cost of $300 a month, and pays the premium through payroll deduction. The second earns $4,000 a month and pays no premium directly. The combined income is $10,000, the shares are 60/40, and the second parent owes $120 of the premium — so the first parent's support obligation is reduced by $120, and the second parent's share of a $4,000 unreimbursed orthodontia bill is $1,600. If instead the first parent earned $4,000 and the second earned $6,000, the shares reverse: the second parent owes $180 of the premium and $2,400 of the orthodontia bill. These are ILLUSTRATIVE contrasts using the published example's figures; they predict no real order's amounts, and no support figure is asserted for either parent.
Illustrative example only — not a fact about any real policy.