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Most states use one of three models to calculate child support. The most common is the Income Shares model, used by the majority of states. It combines both parents' incomes and allocates support proportionally based on the share each parent contributes.

A few states, including Texas and Wisconsin, use the Percentage of Income model, which applies a set percentage of the noncustodial parent's income based on the number of children.

Delaware, Hawaii, and Montana use the Melson Formula, a needs-based approach that first ensures each parent can meet their own basic needs before calculating child support obligations.

While formulas vary by state, common factors include: both parents' gross or net income, the number of children, custody and parenting time arrangements, childcare costs, health insurance premiums for the children, and any special needs of the children. Some states also consider the standard of living the child would have enjoyed had the parents stayed together.

Each state has its own guidelines and worksheets, and judges may deviate from the guidelines in certain circumstances. Income from all sources is typically considered, including wages, bonuses, commissions, and self-employment income.

Child support and alimony (spousal support) are separate obligations. Child support is calculated first in most states and is intended specifically for the children's needs — housing, food, clothing, education, and medical care. Alimony is for the support of a former spouse.

An important distinction: child support is not taxable income for the receiving parent and is not tax-deductible for the paying parent. This differs from alimony rules for agreements finalized before 2019. If you have both obligations, your total support picture will factor in both.

Yes. In most states, child support can be modified when there is a significant change in circumstances. Common reasons include a substantial change in either parent's income, a change in custody or parenting time, changes in the child's needs (such as medical or educational expenses), or changes in childcare costs. Many states allow a review of the order every few years even without a major change.

Income Shares Model

Used by the majority of states, the Income Shares model is based on the concept that children should receive the same proportion of parental income they would have received if the parents lived together. Both parents' incomes are combined, and a basic support obligation is determined from a state-specific guideline table based on that combined income and the number of children.

Each parent's share of the obligation is proportional to their percentage of combined income. The non-custodial parent typically pays their share to the custodial parent. Additional costs like childcare and health insurance for the children are added on top of the base obligation and divided proportionally.

Percentage of Income Model

Used by some states, including Texas and Wisconsin, this model applies a flat percentage of the non-custodial parent's income. Texas uses 20% for one child, 25% for two, 30% for three, 35% for four, and 40% for five or more, based on the non-custodial parent's net resources. Wisconsin uses similar percentages applied to gross income: 17% for one child, 25% for two, 29% for three, 31% for four, and 34% for five or more.

This model is simpler to calculate but does not account for the custodial parent's income, which can produce results that feel unfair in cases where the custodial parent earns significantly more than the non-custodial parent.

Melson Formula

Used in Delaware, Hawaii, and Montana, the Melson Formula is more complex than the other models. It first ensures each parent can meet their own basic living needs (a self-support reserve), then calculates a primary support obligation based on the children's needs, and finally allocates a percentage of any remaining income above those thresholds. This needs-based approach tends to produce lower support amounts for lower-income families and higher amounts when both parents earn well above basic needs.

While the calculation model varies by state, several factors consistently influence the final child support amount:

• Number of children: More children generally means a higher total obligation, though the per-child amount typically decreases with each additional child.
• Each parent's gross and net income: This is the primary driver in all models. Income from all sources is typically considered, including wages, bonuses, commissions, freelance income, and investment income.
• Custody arrangement: Sole custody, joint custody, and split custody all affect the calculation differently. More overnight stays with the non-custodial parent typically reduces the support amount, since that parent is bearing more of the direct costs during those overnights.
• Healthcare and insurance costs: The cost of health insurance premiums for the children is usually added to the base obligation and divided between parents.
• Childcare expenses: Daycare, after-school care, and summer camp costs related to a parent's work schedule are typically included in the calculation.
• Special needs: Medical, educational, or therapeutic needs of the child can increase the support amount beyond the standard guidelines.
• Other children: If either parent supports children from other relationships, this may reduce the income available for the calculation.

Child support guidelines produce a presumptive amount, but judges can deviate upward or downward based on the specific circumstances of the case. Deviations are more common in high-income cases, cases involving special needs children, or situations where the standard guideline would produce an unjust result.

Child support is for the child's needs — housing, food, clothing, education, and medical care. Alimony is for the former spouse's support. They are calculated separately and serve different purposes.

Child support typically has a clear end date: usually when the child turns 18 or graduates high school, though some states extend the obligation to age 19 or 21, and a few states may require support through college. Alimony duration, by contrast, is based on the length of the marriage and varies much more widely.

Child support has been non-taxable to the recipient and non-deductible by the payer for much longer than alimony — this has been the case for decades, well before the 2018 alimony tax change. In most states, child support obligations take priority over alimony: child support is calculated first, and the payment reduces the income available for alimony calculations.

If you have both obligations, estimate your alimony after accounting for child support to see the full picture.

Either parent can petition for a modification if there is a substantial change in circumstances. Common triggers include: job loss or significant income increase, a change in the custody arrangement, a child's changing medical or educational needs, or a significant change in childcare costs. Most states allow modification if income has changed by 10–20% or more since the last order.

You cannot unilaterally stop paying or reduce payments on your own — you must get a court order for the modification. Continuing to pay the court-ordered amount until the modification is approved is critical. Back support (arrears) cannot be discharged in bankruptcy and can result in wage garnishment, driver's license suspension, passport denial, or contempt of court.

If your income changes, act quickly. The modification typically applies from the date you file the petition, not retroactively to when the change in circumstances occurred. Waiting to file means you may owe the original amount for the months between the change and your petition.

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