Two Main Calculation Models
States use two primary models for calculating child support: the Income Shares Model (used by approximately 40 states) and the Percentage of Income Model (used by approximately 10 states). Both attempt to estimate the amount parents would spend on the child if they lived together, then apportion that amount based on each parent's income and custody arrangement.
Income Shares Model — How It Works
The Income Shares model: (1) Calculate both parents' combined gross monthly income; (2) Look up the "basic child support obligation" from the state's guideline table — a fixed amount based on combined income and number of children; (3) Divide this obligation proportionally between the parents based on each parent's percentage of combined income; (4) The non-custodial parent pays their share to the custodial parent. Example: combined income $6,000/month, two children, basic obligation $1,200. Non-custodial parent earns $4,000 (67% of combined) → pays 67% × $1,200 = $804/month.
Percentage of Income Model
Some states (including Texas, Alaska, Nevada, and others) use a simpler Percentage of Income model: a fixed percentage of the non-custodial parent's income based on the number of children. In Texas: 20% for one child, 25% for two, 30% for three, up to 40% for five+. This model is simpler but doesn't account for the custodial parent's income, which critics note can produce inequitable results. The flat percentage applies to net monthly income after taxes and certain deductions.
Factors That Adjust the Amount
Both models allow adjustments for: health insurance premiums (added to or deducted from support depending on who pays); childcare costs (daycare, before/after school care) are typically shared proportionally; extraordinary medical expenses for the child; educational expenses; overnight parenting time — most states reduce the non-custodial parent's obligation when they have significant parenting time (typically a formula kicks in above 30–40% overnight time); other children the non-custodial parent is legally obligated to support; and special needs of the child requiring extra expenses.
When Courts Deviate from Guidelines
Courts can deviate from guideline amounts when application of the guideline would be unjust or inappropriate. Common reasons: extraordinary high or low incomes; children with significant special needs; voluntary unemployment or underemployment; shared custody arrangements not captured by the standard formula; and significant wealth or assets beyond income. Deviations require written findings explaining why the guideline amount is inappropriate.
When to Modify
Either parent can request modification of an existing order when there is a "substantial change in circumstances" — typically a significant change in income (often defined as 15–25% change), change in custody arrangement, or change in the child's needs. See Modifying Child Support Orders.
State Calculators
Every state publishes its child support guidelines and many provide online calculators. Search "[your state] child support calculator" to find your state's official tool. These calculators give a reasonable estimate but not a guarantee — courts have discretion to deviate from guidelines. For a formal calculation, work through your state's child support enforcement agency or an attorney. See How to Apply for Child Support Services.