Child maintenance (CMS) estimator
For a paying parent on £500 a week with 1 child and 52 nights of shared care a year, the Child Maintenance Service basic-rate formula gives an estimated £51 a week, or £2,674 a year. The actual amount is set by the Child Maintenance Service, which uses HMRC income data and can apply variations.
How is child maintenance calculated?
The Child Maintenance Service builds the figure in steps, set out on gov.uk, and this tool follows the same order. The calculation starts with the paying parent's gross weekly income, ignoring anything above £3,000. The rate band is then read off that income. Income up to £100 pays a flat £7. Income between £100 and £200 uses a reduced rate. Income between £200 and £800 uses the basic rate of 12% to 19%. Income up to £3,000 above that uses the basic-plus rate. If the paying parent has other children living with them, the income used for the basic rates is treated as 11% to 16% lower depending on how many, under the Child Support Maintenance Calculation Regulations 2012.
Shared care is the last step in the calculation. The more nights the children stay with the paying parent, the larger the reduction, from one seventh of the amount up to a half, plus an extra £7 a week per child once stays pass 175 nights a year. Both parents see the same figure, because maintenance paid by one parent is maintenance received by the other.
What would child maintenance be, as a worked example?
Take a gross weekly income of £500, one child, no other children at home, and 52 nights of shared care a year. That income sits in the basic band, so the rate is 12%, which comes to £60 a week before any reduction. The 52 nights fall in the one-seventh reduction band. Applying that reduction brings the estimate to about £51 a week, roughly £223 a month. A higher number of nights or additional children in the paying parent's household would each lower the figure further.
Common questions
How is child maintenance calculated?
The Child Maintenance Service works from the paying parent’s gross weekly income. On the basic rate that is 12% for one child, 16% for two and 19% for three or more. Income is first reduced if the payer has other children living with them, and then again for nights the children stay overnight.
What are the basic and basic-plus rates?
The basic rate of 12%, 16% or 19% applies to gross weekly income between £200 and £800. Above £800 the basic-plus rate adds 9%, 12% or 15% on the slice up to £3,000. Income above £3,000 a week is not counted in the standard calculation.
How do overnight stays reduce the amount?
Nights the child spends with the paying parent cut the bill. Between 52 and 103 nights a year reduces it by one seventh, 104 to 155 by two sevenths, and 156 to 174 by three sevenths. At 175 nights or more it is halved and reduced by a further £7 a week for each child.
What if the payer has other children to support?
If the paying parent has other children living in their household, the basic and basic-plus rates are worked out on gross income treated as lower: 11% lower for one other child, 14% for two and 16% for three or more. On the reduced rate, for income between £100 and £200, the regulations use a lower percentage table instead. This tool applies both rules.
Is this the official figure?
No. This is an estimate to show how the rules fit together. The Child Maintenance Service uses your exact income from HMRC and can vary the amount for special circumstances, so treat the gov.uk calculator and any CMS decision as the authority.
What would child maintenance be, as a worked example?
On a gross weekly income of £500 with one child, no other children in the paying parent's household and no shared care, the basic rate of 12% gives an amount before any reduction. Adding a number of shared-care nights then reduces that figure by a fraction that depends on the band those nights fall into. The Child Maintenance Service applies the same steps to the exact income it holds, so the final decision may differ from this estimate.
Sources & Rate Verification
Rates verified against official statutory guidance on 6 September 2026.