Maintenance Calculator
A maintenance calculator translates the UK Child Maintenance Service (CMS) rules into a concrete estimate. You enter a few straightforward numbers – gross income, the children involved, and how often they stay overnight with the paying parent – and the calculator gives an approximate weekly payment. The tool uses the same formula the CMS applies, so you can see what a statutory arrangement might look like before you open a case.
The mandatory calculation method is set out in law. The CMS looks at the paying parent’s gross weekly income (total earnings before tax, National Insurance, and pension contributions, from employment or self‑assessment). It then applies a fixed percentage based on how many qualifying children need support, and adjusts that figure if the paying parent has other dependent children at home. Finally, any shared‑care arrangement (overnight stays with the paying parent) reduces the amount.
How Are Child Maintenance Payments Determined?
Most cases follow the basic rate structure. The gross weekly income is assessed up to £3,000 (as of 2026). The percentages are:
- 12% for one child
- 16% for two children
- 19% for three or more children
If the paying parent receives certain benefits (Income Support, Jobseeker’s Allowance, Pension Credit, etc.) or has a gross weekly income below £100, the flat rate of £7 per week applies regardless of the number of children.
When gross weekly income falls between £100 and £200, a reduced rate is used: a flat £7 plus a small percentage of the income between £100 and £200 (the exact percentage depends on the number of children, typically around 17–25% of that band). The calculator automatically selects the correct band.
After the basic (or reduced) liability is calculated, the total is divided equally among the qualifying children. The CMS then applies a per‑child reduction for any nights the child stays overnight with the paying parent. The bands for shared‑care reductions are:
- Under 52 nights a year – no reduction
- 52–103 nights – 1/7 off the child’s share
- 104–155 nights – 2/7 off
- 156–174 nights – 3/7 off
- 175 or more nights – 1/2 off, plus nil maintenance if the care is truly equal
The calculator runs these numbers behind the scenes once you supply the overnight pattern for each child.
Example: Two Children and Shared Care
A paying parent earns a gross salary of £550 per week and has two children with the former partner. There are no other children in the household. The children each stay with the paying parent 85 nights a year.
- Total basic liability: 16% of £550 = £88
- Each child’s notional share: £44
- Both children fall into the 52–103 night band, so a 1/7 reduction applies to each: £44 − (1/7 × £44) = £37.71 per child
- Weekly total to pay: £75.42
If one child stayed 120 nights and the other 80 nights, the first child’s reduction would be 2/7 and the second’s 1/7, yielding a slightly different combined total.
What Other Factors Can Change the Estimate?
Other children in the paying parent’s home. Before computing the basic rate, the CMS deducts a proportion of gross income for any other biological or adopted children who live with the paying parent. For one other child the deduction is 11%, for two it is 14%, and for three or more it is 16%. The maintenance calculator applies this adjustment as soon as you enter that information.
Income over £3,000 per week. The CMS can, on application, consider income above £3,000 through a variation. A maintenance calculator will typically show only the basic liability up to that cap; if the paying parent’s income exceeds the threshold you may need to ask the CMS for a supplementary top‑up.
Collect & Pay surcharges. The calculator shows the amount the paying parent would pay under a Direct Pay arrangement. If the case is managed through Collect & Pay (where the CMS collects the money), a 20% surcharge is added to the liability and a 4% fee is taken from the receiving parent’s payment. These fees are not part of the statutory formula and are shown separately in CMS statements.
Annual review. All statutory maintenance cases are reviewed yearly. If the paying parent’s income changes by at least 25% between reviews, either parent can request a recalculation. Using the calculator periodically helps you keep an eye on the expected figure.
This calculator provides estimates based on UK Child Maintenance Service formulas as of 2026. It is not a substitute for an official CMS calculation or legal advice. For a binding figure, contact the Child Maintenance Service directly.