How Child Support Is Calculated
Child support is calculated by formula, not by argument. That surprises parents who arrive expecting to make a case about what the children need.
Understanding the formula is worth doing anyway, because it tells you which facts actually move the number — and most of the things people fight hardest about do not.
The two main models
Income shares is used by most states. It estimates what the parents would have spent on the children had they stayed together, based on their combined income, then divides that between them in proportion to what each earns.
Percentage of obligor income takes a set percentage of the paying parent's income, varying with the number of children. Fewer states use it, and it generally does not consider the receiving parent's income at all.
A small number of states use other approaches — Delaware, Hawaii and Montana use a version of the Melson formula, which builds in a self-support reserve for each parent before dividing anything.
Which model your state uses changes the answer substantially. It is the first thing to establish.
What generally goes in
Across models, the recurring inputs are:
- Both parents' incomes, usually gross rather than net, and often including bonuses, commission and self-employment income
- Number of children covered by the order
- Overnights each parent has — in most income-shares states this is a direct input
- Health insurance premiums for the children
- Work-related childcare
- Support paid for children from other relationships
- Sometimes extraordinary medical or educational costs
Why overnights matter more than people expect
In most states the number of nights the children spend with each parent feeds directly into the calculation. Cross a threshold and the number can move sharply.
This has a consequence worth naming: schedule negotiations and support negotiations are the same negotiation, whether or not anyone says so. A parent asking for more overnights may be asking for more time with their children, or may be asking for a lower obligation, and frequently both are true at once.
It also means an informal arrangement that differs from the order can matter. If the order says every other weekend but the children are actually with the other parent half the time, that gap can become relevant — in either direction.
What usually does not move it
- How the money is spent. Support is not itemised and generally does not have to be accounted for.
- A new spouse's income, in most states, though there are exceptions and it can matter indirectly.
- Who was at fault in the separation. Support is about the children, not conduct.
- What the parents privately agreed, if it falls materially below the guideline. Courts frequently decline to approve that, because the obligation is generally understood to belong to the child.
Deviating from the guideline
Every state allows departures from the formula in defined circumstances — a child with extraordinary medical needs, very high or very low parental income, extended travel costs for parenting time.
Deviations generally require the court to make findings explaining them. They are the exception rather than the negotiating position.
Changing it later
Support can usually be modified when circumstances change substantially — a significant income change, a change in the schedule, a child ageing out. Many states also allow periodic review regardless.
The important practical point: modification is generally not retroactive to when your circumstances changed, but to when you filed. A parent who loses a job and waits six months to file usually accrues six months of arrears at the old rate. If your circumstances change materially, that is a reason to move quickly.
Why this page has no numbers
Because any figure would be wrong for most readers.
The formula differs by state, the inputs differ by family, and the output depends on facts specific to you. A number from a website is worse than no number, because it creates an expectation you then negotiate against.
Every state child support agency publishes a calculator or worksheet, and most are free. That is the place to get an estimate. An attorney will tell you what the formula produces for your actual facts, and where a deviation might be argued.
Where Civly fits
Civly does not calculate or handle child support. It is a court-ordered transfer under a state formula, and no app belongs in the middle of it.
What Civly does hold is the two things that feed into support conversations: the overnight record — the schedule as actually lived, not as written — and the shared expense record, with receipts. Both are things parents otherwise reconstruct from memory when a review or modification comes around.
This is general information about how child support formulas generally work, not legal advice. Models, inputs and thresholds vary substantially by state, and nothing here should be relied on for your own case. Contact your state's child support agency, or a family law attorney licensed where you live.