How does the Child Maintenance Service calculate child maintenance?

Last updated: 01/12/2021

After you apply for child maintenance the Child Maintenance Service will contact HM Revenue & Customs to find out the other parent’s income.

CMS Contact Details

Child Maintenance Service 21
Mail Handling Site A
WV98 2BU

Telephone: 0800 171 2345

Relay UK: 18001 then 0800 232 1975

Monday to Friday, 8am to 7.30pm

Saturday, 9am to 4.30pm

Calls are free from mobiles and landlines.

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After you apply for child maintenance the Child Maintenance Service should contact HM Revenue & Customs to find out the other parent’s income. They will also contact the other parent to ask for details about their circumstances and will usually give a calculation of how much child maintenance will be payable, based on the information received from HM Revenue & Customs.

There are certain things which affect a paying parent’s income that can be considered by the Child Maintenance Service when calculating child maintenance payments. These include payments into pension schemes, certain costs or expenses the paying parent pays, and the number of other children who the paying parent supports.

If the paying parent is a student earnings from employment will be used to calculate child maintenance payments but students finance, including loans and grants, will not count.

Child maintenance is reviewed each year. Payments may be affected by changes in the paying parent’s circumstances, such as a change in their income or family circumstances. The Child Maintenance Service should be told about these changes as soon as possible.

If the paying parent, their employer, or accountant has not supplied gross income information to HM Revenue & Customs for use in making a child maintenance decision, the Child Maintenance Service can ask for this information.

Child maintenance rates

Child maintenance rates are applied to gross weekly income. The rates differ depending on the gross weekly income amount:

Best evidence assessment

If the Child Maintenance Service do not get the information they need to calculate the maintenance payment they can either make a “best evidence assessment” or a “default maintenance decision”.

A “best evidence assessment” uses previous information held about a paying parent’s gross income to work out an amount of child maintenance that must be paid.

Default maintenance decision

A “default maintenance decision” is made where there is no information about the paying parent’s income. It is a fixed amount based on the number of children the paying parent must pay child maintenance for.

The default rates are:

£38 a week for one child
£51 a week for two children
£61 a week for three or more children

Basic rate

The Basic rate applies if the paying parent’s gross weekly income is between £200 and £3000.

The following percentages of gross income will apply:

12% for one qualifying child
16% for two qualifying children
19% for three or more qualifying children

If the paying parents gross weekly income is over £3000 per week you can apply to the court for extra child maintenance.

Reduced rate

A reduced rate is applied to paying parents with a gross weekly income of more than £100 but less than £200.

The Reduced rate consists of two parts:

  • An amount equal to the flat rate. This currently stands at £7 per week plus
  • A percentage of the balance of gross weekly income above £100

The pecentages are:

17% for one qualifying child
25% for two qualifying children
31% for three or more qualifying children


If a paying parent’s income is £110 per week and there is one qualifying child, the child maintenance payment would be:

The flat rate of £7 plus 17% of £10 (the weekly income over £100)

£7 + £1.70 = £8.70

Flat rate

For gross weekly income of £100 or less, or where a paying parent receives an income based benefit, a Flat rate of £7 applies, regardless of the number of qualifying children.

Nil rate

A Nil rate applies if the paying parent is under 16, under 19 and in full-time education, a prisoner or if the gross weekly income is less than the flat rate amount.


Reductions for other children

If your child’s other parent is paying maintenance for children from another relationship the amount of maintenance you receive will be reduced.

If your children’s other parent has a child from another relationship living with them (a relevant child) this will also reduce your maintenance payments.

If you do not agree with the child maintenance calculation

If you think your child maintenance calculation is wrong you can ask for a revision, supersession, variation or can appeal. Which one you ask for depends on what you think is wrong.

Revisions, supersessions and variations


If you think the Child Maintenance Services has made a mistake in your child maintenance calculation you can ask them to look at it again. This is called a revision or a mandatory reconsideration. The mistake could be because they got the number of children wrong or did not use the correct income.

Depending on what is found in the revision you could then go on to ask for a supersession or a variation.

Details of how to ask for a revision will be included in the letter you received about the calculation.



If there is a change in circumstances after your child maintenance calculation has been made you can ask for a supersession. This could be because of something like a change in the paying parent’s income or your child leaving school to start university.

The new calculation can be backdated to the time of the change.

To apply for a supersession contact the Child Maintenance Service by phone or in writing.



If you are the parent receiving child maintenance you can apply for a variation if you think the income used in the calculation is wrong. If you believe the parent paying maintenance has income they have not declared you can ask that the Child Maintenance Service take this into account. The Child Maintenance Service need proof of this other income before they can make the variation. Unfortunately this can be difficult to get  in some circumstances.

If you are the parent paying maintenance you can also ask for a variation. Certain expenses can be taken into account and could reduce the amount of child maintenance you have to pay. These include:-

  • Contact costs
  • Costs of a long-term disability or illness of a “relevant child”
  • Debts of the relationship
  • Boarding school fees

Contact the Child Maintenance Service by phone or in writing to apply for a variation. Give as much information as you can about the other parents income and keep a note of what you have said or a copy of the letter you have sent.



If you are unhappy with a decision the Child Maintenance Service has made, on a supersession for example, you can appeal.  You must first ask them to look at the decision again (revision) but if you are still not happy you can then appeal.

An appeal is carried out by a tribunal that is not part of the Child Maintenance Service. You must appeal within one month of receiving the result of the mandatory reconsideration.

Details of how to appeal will be in the letter you receive with the result of the mandatory reconsideration. You may need help from a solicitor or other legal service to do this.

If the paying parent lives abroad

If your child’s other parent lives abroad you can still use the Child Maintenance Service if your child’s other parent is:

  • A UK civil servant or works within Her Majesty’s Diplomatic Service
  • A member of the armed forces
  • Working for a company that is based and registered in the UK
  • Working on secondment for a UK regional health authority or local council.

If your child’s other parent does not fit into one of these you can ask the Scottish Government for advice by emailing, phoning or writing to them at:


Telephone: 0131 244 2417 or 0131 244 4829

Fax: 0131 244 4848

The Scottish Government Justice Directorate
Central Authority and International Law Team
St Andrew’s House
Regent Road