CSA Assessment Calculator

When the Child Maintenance Service (CMS) issues a Maintenance Calculation Notice, it applies the statutory formulas set out in Schedule 1 to the Child Support Act 1991 (as substituted by the Child Maintenance and Other Payments Act 2008). The calculation begins with the paying parent's gross weekly taxable income, verified directly against HMRC Real Time Information (RTI).

Key adjustments applied to the gross figure before rate calculation include:

• Registered Pension Contributions: 100% relief is granted for occupational, personal, or stakeholder pension contributions. If contributions are paid via salary sacrifice or net pay, they are already reflected in taxable gross; if paid relief-at-source, grossed-up values must be submitted manually.
• Relevant Other Children (ROC): If the paying parent or their cohabiting partner receives Child Benefit for children resident in their current household, gross income is reduced by 11% (1 child), 14% (2 children), or 16% (3 or more children).
• Shared Care Bands: If qualifying children stay overnight with the paying parent for 52 or more nights annually, statutory reductions of 1/7th, 2/7th, 3/7th, or 50% plus £7 per child per week apply.

Under Regulation 34 of the Child Support (Maintenance Calculation) Regulations 2012, the CMS will only replace historic tax year figures with current income if the change is at least 25%. If a paying parent's income drops by 20% due to a pay cut or reduced overtime, the CMS will continue assessing liability based on the higher historic earnings.

Conversely, if a parent's income increases by 24%, the receiving parent cannot trigger an in-year upward revision unless a fresh annual review is reached or total income exceeds the 25% threshold.

If either parent believes the assessment contains factual errors—such as incorrect overnight bandings, omitted pension contributions, or erroneous HMRC earnings records—they must submit a request for Mandatory Reconsideration within 30 days of the notice date.

The CMS review team re-examines the original decision. If the reconsideration is refused or upheld unchanged, the parent receives a Mandatory Reconsideration Notice (MRN), which unlocks the legal right to lodge an appeal with HM Courts & Tribunals Service (Social Security and Child Support First-tier Tribunal).