How child maintenance amounts are decided
July 2026
Most guidance on child maintenance — known as child support in some countries — starts and ends with a calculator: enter an income, get a number. What the calculators rarely explain is the logic behind that number, or why two similar-looking families can end up with different figures. Understanding the mechanics does not make the amount less real, but it does make disagreements about it easier to have, because most conflict is not about whether to pay; it is about whether the formula has accounted for the actual arrangement.
Three ways to calculate the number
Child maintenance systems generally run on one of three underlying logics, though most combine elements of more than one. The first is a percentage-of-income model: a set share of the paying parent's gross income, rising with the number of children. The UK's Child Maintenance Service works this way: gross weekly income is assessed from HMRC records, then one of several rate bands applies, with the percentage rising for each additional child.
The second is an income-shares model, the dominant approach across many US states. Both parents' incomes are combined to estimate what the child would have received had the household stayed together, and each parent is then responsible for a proportional share of that total.
The third is a published table or needs-based model, where an amount is set by the child's age and the paying parent's income bracket, sometimes adjusted for specific circumstances. Norway's system runs on this logic: a national baseline cost of raising a child, set by age band, adjusted from there. None of these produces the correct number; each is a policy choice about how to standardise something that used to be negotiated case by case.
What actually feeds into the formula
Income is the anchor everywhere, but the adjustments around it are where most of the real variation sits. Almost every system now includes some form of shared-care adjustment. The UK reduces the liability once a child stays overnight with the paying parent at least 52 nights a year, on a sliding scale. Norway's contact deduction works in four bands, from two to three nights a month up to fourteen or more, calculated from the pattern actually followed rather than the one written into an agreement.
Most systems also reduce the assessable income if the paying parent is financially responsible for other children, whether from a new relationship or an earlier one. And several table-based systems, Norway's included, raise the baseline amount as the child gets older, on the reasoning that the actual cost of raising a child rises with age.
What maintenance covers, and what it does not
Maintenance is designed to cover a child's ordinary day-to-day costs (food, a housing share, basic clothing), not the full range of costs a child generates over a year. Activities, school trips, larger equipment, and one-off purchases typically sit outside it, which is why they need a separate agreement between parents rather than an assumption that the maintenance figure already covers them. For that side of the ledger, see Splitting expenses across two homes.
When the amount changes
Maintenance is rarely a one-time calculation. Most systems allow, and some require, a review when circumstances shift materially: a significant change in either parent's income, a change in the actual overnight pattern, or the child reaching an age band that changes the baseline.
Reviews are typically triggered by request rather than automatically. In practice, this means a parent whose situation has changed generally has to initiate the process rather than expect the system to catch up on its own.
When the number becomes the fight
According to National Family Mediation, maintenance disputes rarely start as a refusal to pay. They start with a parent believing the assessment does not reflect reality: an income that has changed, care time that increased without a review, or additional costs one parent feels the other should share.
Statutory agencies such as the UK's Child Maintenance Service have real enforcement powers for genuine non-payment, including deductions from earnings. But a mismatch between the formula and the actual arrangement is usually resolved faster through a review request or mediation than through escalation.
This is where documentation quietly matters. When a shared-care adjustment depends on the number of overnights, a clear record of the pattern both parents have actually followed supports a review request far better than two differing memories of what was agreed. Apps built for shared-care coordination, including Lina, keep that record in one place both parents can see, not as a replacement for the formal process, but as the shared set of facts a review or mediation session needs to start from.
What the formula cannot decide
The formulas exist to make maintenance predictable rather than personal: to turn a conversation that used to happen court by court, family by family, into a calculation. That is a real gain: fewer disputes are settled by whoever argues more persistently.
What the formulas cannot standardise is a family's sense of whether the number is fair for their specific arrangement. That gap tends to close not through a better formula, but through parents who can point to the same facts about what is actually happening.
Sources
GOV.UK: How child maintenance is worked out →
House of Commons Library: How is child maintenance calculated? →
Bufdir: Barnebidrag, barnetrygd og andre stønader ved samlivsbrudd →
OECD Family Database: PF1.5 Child Maintenance (Child Support) →
Common questions
Is child maintenance the same as child support?
The terms describe the same thing. Child maintenance is used in the UK and several other countries; child support is more common in the US and in international comparisons.
Does the number of overnight stays affect the amount?
In most systems, yes. Once a child spends a set number of nights per year with the paying parent, the liability is typically reduced on a sliding scale, since that parent is directly covering more of the child's costs during that time.
What happens if the other parent stops paying?
Statutory agencies generally have enforcement powers, such as deductions from wages, for genuine non-payment. If the dispute is really about whether the assessment is correct, a review request or mediation is usually the faster route.
Related articles
Document the pattern, not just the plan
When a shared-care adjustment depends on the actual number of overnights, Lina's care schedule gives both parents the same record of what happened, useful when a review request needs facts, not a memory.
Open the care schedule