When a paying parent has qualifying children with more than one former partner, establishing the correct child maintenance liability requires the application of the statutory CMS Apportionment Formula.

Under Regulation 45 of the Child Support Maintenance Calculation Regulations 2012 and Part IV of Schedule 1 to the Child Support Act 1991, the law prevents double-charging by treating all qualifying children as a unified group before dividing the liability equitably among the receiving parents.

The 5-Step Statutory Apportionment Mechanism

The CMS applies a sequential 5-step statutory calculation process:

Step 1: Establish Total Assessable Weekly Income

The paying parent's gross weekly taxable income is verified via HMRC Real-Time Information (RTI) records, minus allowable registered pension deductions (Regulation 37).

Step 2: Deduct Resident Children in the Paying Parent's Current Household

If the paying parent lives with children in their new household (their own biological children or step-children for whom their current partner receives Child Benefit), a statutory percentage deduction is made:

  • 1 resident child: 11% gross income discount
  • 2 resident children: 14% gross income discount
  • 3+ resident children: 16% gross income discount

Step 3: Calculate the Aggregate Maintenance Liability

The CMS counts the total number of qualifying children across all receiving households. For example, if Parent A has 1 child and Parent B has 2 children, the total qualifying child count is 3 children.

  • The statutory rate applied is the 3-child rate (19% on Basic Rate band, or Basic Plus tiered formula).
  • This establishes the Total Family Pool Liability.

Step 4: Equal Per-Child Apportionment

The aggregate maintenance figure is divided equally by the total number of qualifying children:

\text{Per-Child Liability} = \frac{\text{Total Family Pool Liability}}{\text{Total Qualifying Children Count}}

  • Parent A receives 1 share (\frac{1}{3}\text{rd}).
  • Parent B receives 2 shares (\frac{2}{3}\text{rds}).

Step 5: Household-Specific Shared Care Deductions (Regulation 46)

Once the per-child baseline liability is apportioned, overnight shared care discounts are calculated independently for each receiving household based on the actual nights spent with the paying parent:

  • Alternate weekends (52–103 nights) = 1/7th deduction.
  • 2 nights per week (104–155 nights) = 2/7ths deduction.
  • Equal care (175+ nights) = 50% discount plus £7 per child statutory deduction.

Comprehensive Case Example

Consider a paying parent with £750.00 gross weekly income (£39,000/year), no resident children:

  • Child 1 lives with Mother A (alternate weekend shared care, 1/7th discount).
  • Child 2 & Child 3 live with Mother B (no overnight shared care).
  1. Total Children: 3 children (Basic Rate tier = 19%).
  2. Gross Combined Liability: 19% of £750 = £142.50 per week.
  3. Apportionment per child: £142.50 / 3 = £47.50 per child.
  4. Mother A Calculation:
  5. 1 child share = £47.50
  6. Less 1/7th shared care discount (£47.50 / 7 = £6.79)
  7. Payable to Mother A: £40.71 per week.
  8. Mother B Calculation:
  9. 2 child shares = £95.00
  10. Shared care discount: £0.00
  11. Payable to Mother B: £95.00 per week.
  12. Total Weekly Liability: £40.71 + £95.00 = £135.71 per week.