Child Maintenance Calculator
Estimate your Child Maintenance Service payment using the official CMS formula — 12%, 16% or 19% of gross weekly income, adjusted for nights the children stay with you and any other children you support.
Child maintenance calculator
Based on the CMS 2012-scheme gross-income formula.
An estimate using the published CMS formula. For an official figure use the calculator on GOV.UK.
How the Child Maintenance Service works out payments
The Child Maintenance Service (CMS) uses a set formula based on the paying parent's gross weekly income — their income before tax and National Insurance but after any pension contributions. HMRC supplies this figure directly. The rate that applies depends on how much they earn each week.
On the basic rate (gross income £200–£800 a week) you pay 12% for one child, 16% for two and 19% for three or more. So a parent on £500 a week with two children pays about £80 a week before any shared-care reduction.
The five CMS income bands
| Rate | Gross weekly income | What you pay |
|---|---|---|
| Nil | Below £7 | £0 |
| Flat | £7 to £100, or on benefits | £7 a week |
| Reduced | £100.01 to £199.99 | £7 + a sliding percentage |
| Basic | £200 to £800 | 12% / 16% / 19% |
| Basic plus | £800.01 to £3,000 | +9% / 12% / 15% on the part above £800 |
Shared care reduces the amount
If the children stay overnight with the paying parent, the maintenance is cut on a sliding scale: 52–103 nights a year reduces it by one-seventh, 104–155 nights by two-sevenths, 156–174 nights by three-sevenths, and 175 nights or more halves it and then takes off a further £7 per child per week.
Other children matter too. Before the percentage is worked out, the paying parent's gross income is reduced for any other children they support — by 11% for one, 14% for two, and 16% for three or more.
Family-based arrangements
Many separated parents avoid the CMS altogether with a private "family-based arrangement", agreeing an amount between themselves. It's free, flexible and avoids CMS collection fees. The figures here are a useful starting point for that conversation. If you do use the CMS, full guidance is on GOV.UK.
Frequently asked questions
How is child maintenance calculated by the CMS?
The CMS uses the paying parent's gross weekly income (before tax, after pension). On the basic rate (£200–£800 a week) you pay 12% for one child, 16% for two and 19% for three or more. Income above £800 up to £3,000 is charged at the lower basic-plus rates of 9%, 12% and 15%.
Does shared care reduce child maintenance?
Yes. 52–103 nights a year cuts it by 1/7, 104–155 nights by 2/7, 156–174 nights by 3/7, and 175 or more nights halves it and removes a further £7 per child.
What is the flat rate of child maintenance?
If the paying parent's gross weekly income is £100 or less, or they receive certain benefits, the flat rate of £7 a week applies regardless of the number of children.
Do other children in the household affect the amount?
Yes. Before the percentage is applied, gross income is reduced for other children the paying parent supports — by 11% for one, 14% for two and 16% for three or more.
Is the CMS calculator official?
This is an independent estimate using the published CMS formula. For an official calculation use the GOV.UK child maintenance calculator, which draws income directly from HMRC.