Child Maintenance Service (CMS) Calculator
How to Use This Calculator
Use gross weekly income — before tax and NI. Annual salary ÷ 52 = weekly income.
Children under 16, or under 20 in full-time education, count as qualifying children.
If the child stays with the paying parent, maintenance is reduced by a percentage.
CMS calculates weekly — multiply by 52/12 for the monthly equivalent.
Frequently Asked Questions
How does the CMS calculate child maintenance?
CMS uses the paying parent's gross income. The basic rate is 12% for 1 child, 16% for 2 children, or 19% for 3+ children. This is then reduced for shared care and for other children the paying parent supports.
What income does CMS use?
CMS uses gross income (before tax and NI) from HMRC records. It includes employment income, self-employment, and pension income. Benefits and tax credits are excluded.
How does shared care reduce maintenance?
If the child spends 1-52 nights/year with the paying parent (under 1/week average), a flat-rate reduction applies. 52-103 nights (1-2/week) = 1/7 reduction. 104-155 nights = 2/7 reduction. 156-174 nights = 3/7 reduction. 175+ nights = 50% reduction.
What is the flat rate?
Parents earning under £100/week (or on certain benefits) pay a flat rate of £7/week per case, regardless of the number of children.
Can maintenance be arranged privately?
Yes — a 'family-based arrangement' is always an option and avoids CMS fees. If using the CMS collection service, the paying parent pays an extra 20% fee and the receiving parent loses 4%.
What if income changes?
CMS reviews income annually using HMRC records. Either parent can request a recalculation if income changes by 25% or more. Voluntary income changes (such as pension contributions) may be taken into account.
Are school fees or extras included?
No — the CMS calculation covers standard child maintenance only. Additional costs like school fees, activities, or clothing are separate and must be agreed privately.
What if the paying parent is self-employed?
Self-employed income is assessed based on HMRC tax returns. CMS can look behind declared income if it appears artificially low. In some cases, CMS uses a default income if records are unavailable.