Every CMS calculation follows the same four steps: work out the paying parent’s income, apply the right rate, allow for other children, then reduce for shared care. If you understand those four steps you can check any calculation letter yourself.
Step 1: gross weekly income
The CMS uses the paying parent’s gross income: before tax and National Insurance. It takes off pension contributions.
- Historic income. The CMS normally gets the figure from HMRC for the latest tax year available. For an employee this is close to the P60 figure.
- Current income. If the paying parent’s income now is different from the HMRC figure by 25% or more, the CMS uses current income instead. Either parent can ask for this, with evidence such as recent payslips. The government has announced this threshold will fall to 15%. Check GOV.UK for the date it applies.
- Self-employed income. Taken from the tax return: profits, not turnover.
- Unearned income. Rent, interest and dividends over £2,500 a year are not included automatically. The receiving parent can ask for them to be added through a variation.
The yearly figure is divided by 52 (or by 365 and multiplied by 7) to get a weekly figure.
Step 2: the rate
| Gross weekly income | Rate | How it works |
|---|---|---|
| Under £7, or in some special situations | Nil | Nothing to pay |
| £7 to £100, or getting certain benefits | Flat rate | £7 a week |
| £100.01 to £199.99 | Reduced rate | £7, plus a percentage of income above £100 |
| £200 to £800 | Basic rate | 12% for one child, 16% for two, 19% for three or more |
| £800.01 to £3,000 | Basic plus | Basic rate on the first £800, then 9%, 12% or 15% on the rest |
| Over £3,000 | Capped | Income above £3,000 a week is ignored. The receiving parent can ask a court for more. |
Table: Rates under the 2012 scheme, as at October 2026.
The flat rate applies to paying parents who get benefits such as Universal Credit (where there are no earnings), State Pension, Pension Credit, income-based Jobseeker’s Allowance or income-related Employment and Support Allowance.
Step 3: other children
If the paying parent, or their partner, gets Child Benefit for other children living with them, their income is reduced before the percentage is applied:
| Other children | Reduction to income |
|---|---|
| 1 | 11% |
| 2 | 14% |
| 3 or more | 16% |
On the reduced rate, other children change the percentage used instead. The maintenance estimator does this for you.
Step 4: shared care
If a child stays overnight with the paying parent for at least 52 nights a year, the amount goes down.
| Nights a year with the paying parent | Reduction |
|---|---|
| 52 to 103 | One-seventh |
| 104 to 155 | Two-sevenths |
| 156 to 174 | Three-sevenths |
| 175 or more | Half, plus an extra £7 a week for each child in this band |
Where there is more than one child, the CMS adds up the fractions for each child and divides by the number of children. Shared care cannot reduce the amount below £7 a week. There is much more on this in the shared care guide.
Worked example
A paying parent earns £36,400 a year gross and pays £1,820 into a personal pension. They have one child with the receiving parent and no other children. The child stays with them every other weekend, Friday and Saturday nights, which is 52 nights a year.
| Step | Working | Amount |
|---|---|---|
| Gross weekly income | (£36,400 − £1,820) ÷ 52 | £665.00 |
| Basic rate, one child | £665.00 × 12% | £79.80 |
| Shared care, 52 to 103 nights | £79.80 − one-seventh | £68.40 a week |
Count carefully near the boundaries
Alternate weekends of two nights is exactly 52 nights a year. One missed weekend could drop below the first band, and one extra night each holiday could move a family up a band. The care nights counter shows how close you are.
Checking your calculation letter
The letter sets out each step. Check:
- The income figure and which tax year it comes from
- Whether pension contributions have been taken off
- The number of children in the calculation and their dates of birth
- The number of other children recorded
- The number of nights of shared care for each child, and which band was used
- The effective date
- Whether the rate type (basic, reduced, flat) matches the income
If any of these are wrong, you can ask for a mandatory reconsideration. If something has changed since the calculation, report a change of circumstances instead.
Common questions
Why is the income figure from an old tax year?
The CMS uses the latest tax year HMRC has when it makes the decision. If income has changed by 25% or more since then, ask for current income to be used and send evidence.
Is overtime and bonus included?
Yes, if it is in the taxable pay HMRC reports. If current income is used, the CMS looks at a representative period.
My ex has gone self-employed and declares almost nothing. What can I do?
Ask for a variation on the grounds of diverted income or unearned income, and give the CMS as much detail as you can. See variations.
What happens if the paying parent will not give income details?
The CMS can make a default maintenance decision at a set weekly amount (currently £38 for one child, £51 for two and £64 for three or more) until it gets the information.
Sources and law
- Child Support Act 1991, Schedule 1 (rates and other children)
- Child Support Maintenance Calculation Regulations 2012 (SI 2012/2677), regulations 34 to 45
- GOV.UK: How child maintenance is worked out
