How is child support actually calculated — is it just a guess by the judge?

Illustrative — not a real business

It is not a judge's personal guess — nearly every state uses a specific guideline formula, and while the exact formula varies by state, most weigh both parents' gross incomes and the actual amount of parenting time each parent has, since more overnight time with a child generally corresponds to a lower support obligation for that parent. Courts can deviate from the formula's result, but only with a specific, documented justification, such as a child's extraordinary medical need or a significant income disparity the formula does not adequately capture — deviation is the exception, not the norm. On top of the base calculated amount, many orders also address specific added costs like healthcare premiums, uninsured medical expenses, and sometimes childcare or agreed-upon extracurricular costs, split according to a formula or percentage the parents agree to or the court orders. If either parent's income or the parenting schedule changes significantly down the line — a job loss, a substantial raise, a change in custody arrangement — either parent can request a modification, so the original number is not necessarily permanent for the life of the order.