Our calculator is designed to provide indicative child maintenance estimates for selected circumstances using published UK Child Maintenance Service calculation rules. This page explains the general formulas, key assumptions and limitations. It does not replace an official CMS assessment.
1. Calculation Methodology Overview
Child Maintenance Calculator UK is an independent informational website that helps parents estimate potential child maintenance payments.
The calculator uses information entered by the visitor to estimate a weekly payment. It also presents monthly and yearly equivalents.
The general calculation considers the paying parent's income, qualifying children, permitted adjustments and applicable overnight shared care.
Relevant earnings before Income Tax and National Insurance, subject to permitted adjustments.
The number of children included in the maintenance calculation.
The applicable income band and number-of-children percentage.
Relevant reductions for qualifying overnight stays.
Different circumstances can require additional information or an official case-specific assessment.
2. The Six CMS Calculation Steps
Official CMS guidance describes a six-step process for working out child maintenance.
Identify the paying parent's relevant gross income or applicable benefit circumstances.
Account for relevant pension contributions and other supported children.
Determine whether Nil, Flat, Reduced, Basic or Basic Plus applies.
Determine how many children are included in the calculation, including applicable arrangements.
Apply the relevant statutory rate and percentage.
Consider eligible overnight care and related reductions.
The complete official process can involve additional conditions beyond a simplified online estimate.
3. Gross Income and Weekly Conversion
The CMS generally starts with relevant gross income and may use information obtained from HMRC.
Our calculator accepts weekly, monthly or annual income entries and converts them to an indicative weekly figure.
An annualisation convention is used by this estimator. Official calculations may involve additional processing and rounding rules.
Example: Annual Income
A paying parent reports gross annual income of £31,200.
£31,200 × 7 ÷ 365 = approximately £598.36 per week.
Visitors should enter the paying parent's income, not the combined income of both parents.
Income provided directly by a user may differ from income used in an official CMS assessment.
4. Pension Contributions
Certain pension contributions can reduce the income used for child maintenance purposes.
Employer pension contributions deducted before taxable income is reported may already be reflected in the income figure.
Eligible private pension contributions may need to be accounted for separately, depending on how they were paid.
Only eligible contributions not already reflected in the entered income should be deducted.
Pension Adjustment Example
Gross weekly income: £600
Eligible additional pension contributions: £50
£600 − £50 = £550
Pension tax relief and the contribution method can affect the appropriate adjustment. Complex cases should be verified with the CMS.
5. Other Children Supported
The CMS can take account of certain other children the paying parent supports.
Under applicable rules, eligible other children can reduce the gross weekly income used for an ordinary Basic or Basic Plus calculation.
| Other Supported Children | Income Reduction |
|---|---|
| None | 0% |
| 1 child | 11% |
| 2 children | 14% |
| 3 or more children | 16% |
Other-Child Adjustment Example
Gross weekly income: £600
Other eligible supported children: 1
£600 × 11% = £66 reduction
£600 − £66 = £534
Reduced-rate cases use different percentages that directly affect the maintenance formula. A child must meet the applicable CMS requirements to count in this category.
6. Child Maintenance Service Rates
The general rate depends on the relevant gross weekly income and circumstances of the paying parent.
| Weekly Income | CMS Rate | General Method |
|---|---|---|
| Below £7 | Nil | £0 |
| £7–£100 | Flat | Generally £7 weekly |
| £100.01–£199.99 | Reduced | £7 plus the applicable percentage above £100 |
| £200–£800 | Basic | A percentage of relevant weekly income |
| £800.01–£3,000 | Basic Plus | Two income bands |
The CMS generally uses a maximum of £3,000 gross weekly income for its standard income-based calculation.
Additional maintenance may be considered through a court in qualifying situations.
Qualifying benefits and other special circumstances can change the rate.
Read Full CMS Rates Guide →7. Basic Rate Calculation Formula
For standard Basic rate cases, the percentage depends on the number of children needing maintenance.
| Children | Basic Rate |
|---|---|
| 1 child | 12% |
| 2 children | 16% |
| 3 or more children | 19% |
Basic Rate — Two Children
Relevant gross weekly income: £600
Children needing maintenance: 2
Applicable Basic rate: 16%
£600 × 0.16 = £96
8. Basic Plus Calculation Formula
For relevant gross weekly income above £800, the Basic Plus method uses two bands.
| Children | First £800 | Income Above £800 |
|---|---|---|
| 1 child | 12% | 9% |
| 2 children | 16% | 12% |
| 3 or more | 19% | 15% |
The additional income band is subject to the normal £3,000 weekly income ceiling.
Basic Plus — Two Children
Relevant gross weekly income: £1,000
First £800 × 16% = £128
Remaining £200 × 12% = £24
Total = £128 + £24
9. Reduced, Flat and Nil Rates
Reduced Rate
The general Reduced rate applies to relevant income above £100 but below £200 per week.
The percentage depends on the number of qualifying children and any eligible other supported children.
| Children Needing Maintenance | No Other Supported Children |
|---|---|
| 1 child | 17% |
| 2 children | 25% |
| 3 or more | 31% |
When there are other supported children, the percentage is different. The calculator uses a separate reduced-rate lookup table for those cases.
Reduced Rate — One Child
Gross weekly income: £150
£150 − £100 = £50
£50 × 17% = £8.50
£7 + £8.50 = £15.50
Flat Rate
The general income-based Flat rate is £7 per week.
Some qualifying benefits can also result in a Flat rate, but special rules may apply.
Nil Rate
The general Nil rate is £0. It can apply to income below £7 per week and certain other circumstances.
11. Weekly, Monthly and Yearly Estimates
Our calculator produces a weekly maintenance estimate and converts it into monthly and yearly equivalents.
Payment Conversion Example
Estimated weekly maintenance: £96
Yearly: £96 × 365 ÷ 7 = £5,005.71
Average monthly: £5,005.71 ÷ 12 = £417.14
These figures are mathematical equivalents used for estimation. They do not establish an official payment schedule or collection amount.
12. Additional Worked Examples
These examples assume straightforward income-based cases, with no other supported children, pension adjustment or shared care unless stated.
Example A: One Child
Relevant gross weekly income: £500
Basic rate: 12%
£500 × 12% = £60
Example B: Two Children
Relevant gross weekly income: £750
Basic rate: 16%
£750 × 16% = £120
Example C: Three Children
Relevant gross weekly income: £600
Basic rate: 19%
£600 × 19% = £114
Actual assessments may differ because of official income information, rounding, variations and other case-specific rules.
13. Current Calculator Limitations
The website calculator does not aim to reproduce every statutory CMS calculation scenario.
Some situations require further official assessment or separate calculation procedures.
- Qualifying benefits and associated special rules.
- Equal day-to-day care arrangements.
- Overnight shared care of 175 or more nights.
- Income exceeding £3,000 gross weekly.
- Other-child adjustments that move income into a different calculation band.
- Multiple receiving parents and certain family-based arrangements.
- Complex self-employment, income variations and special expenses.
- Local authority overnight care.
- Court orders and international maintenance situations.
The calculator cannot determine every legal or financial circumstance. A result must not be treated as an official assessment or relied on alone to change existing payment obligations.
Visitors should use the official CMS calculator where they need further guidance.
14. Calculation Testing and Updates
We aim to review our calculator when relevant statutory rules, official guidance or website functionality change.
Appropriate verification should include tests for common scenarios and relevant boundary conditions.
- Each supported income band.
- One, two and three or more children.
- Different income input frequencies.
- Eligible pension adjustments.
- Other supported children.
- Supported overnight stay bands.
- Unsupported or complex circumstances.
- Weekly-to-monthly and yearly conversions.
Representative outputs should be compared with official guidance and, where suitable, the GOV.UK calculator.
Such comparisons help identify possible problems but do not constitute certification by the UK Government.
Errors or discrepancies can be reported using our contact page. See our Editorial Policy for our intended review and correction standards.
15. Official Information Sources
The following official resources explain the UK child maintenance rules relevant to this methodology.
General six-step CMS calculation overview.
Read Official Guidance →Income adjustments, rates and calculation examples.
Read Detailed Guide →Overnight shared care and equal day-to-day care guidance.
Read Shared Care Guidance →Government calculator for indicative child maintenance payments.
Open Official Calculator →Source transparency: Our website is independent. Links to official resources are provided for verification and do not imply government affiliation.
16. Questions and Calculation Feedback
If you identify a possible calculation discrepancy or an unclear explanation, you can contact the website operator.
When reporting a calculation issue, it may help to provide:
- The relevant calculator or page URL.
- A general description of the scenario.
- The displayed result and the expected calculation.
- A relevant official source, if available.
Avoid providing unnecessary identifying information about children or sensitive financial records through general enquiries.
Website: child-maintenance-calculator.co.uk
Website Operator: [INSERT ACTUAL BUSINESS OR LEGAL NAME]
Email: [INSERT VERIFIED CONTACT EMAIL]
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Use the free homepage calculator to estimate weekly, monthly and yearly maintenance payments for supported scenarios.
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