The 'Mostyn formula' - Adjusted Formula Methodology (AFM)

Where there is a claim for child maintenance and the paying party’s income exceeds £156,000, the maximum level assessable under the CMS formula, the receiving party can apply for extra maintenance through the courts. This is referred to as a ‘top-up’ order. 

In order to provide clarity as to how much extra child maintenance should be awarded in such cases, Mostyn J has set out his own ‘Adjusted Formula Methodology’, which has also been called the ‘Mostyn formula’.  

In TW & TM (Minors), Re [2015] EWHC 3054 (Fam), Mostyn J held that where the paying parent’s income exceeds the CMS threshold but is not “unadjacent” to it, the standard CMS formula may still be useful. Furthermore, in CB v KB [2019] EWFC 78 he set out that the starting point of the court should be to apply the CMS formula in cases where the paying parent’s income is below £650,000. 

However, this approach faced some criticism, particularly over it producing very high figures, especially in cases where there was only one or two children. Notably, Moor J in CMX v EJX (French Marriage Contract) [2022] EWFC 136 highlighted the issues arising from the broad application of this approach. Moor J stated that whilst he accepted the “beauty” of the approach, in that it provided an easy calculation and potentially helped avoid dispute, it also resulted in unrealistic awards of child maintenance for high earners, as well as substantial disparities between awards for a single child, compared to awards for three.  

Mostyn J accepted Moor J’s criticism, noting that where the paying parent’s income in £650,000 the CMS formula would result in an award of £60,000 for a single child, and £33,000 for each of three children. As stated by Mostyn J, this is problematic given that one child plainly does not cost 80% more than one of a trio of children. To address these issues, in James v Seymour [2023] EWHC 844 (Fam) Mostyn set out his own ‘Adjusted Formula Methodology’ (AFM) to be applied instead of the CMS formula. This formula provides a ‘Child Support Starting Point’ (CSSP), which is a “loose starting point” to aid in the determination of child maintenance. Mostyn helpfully attached a table to his judgment, outlining the application of the AFM.

As emphasised by Mostyn J, calculating the sum of child maintenance payable requires a discretionary balancing exercise, and the court must look at the relevant budget in each case. As such, judges are free to accept or dismiss the sum calculated by the AFM.  

Furthermore, Mostyn J clarified that the AFM should not be used in the following cases where: 

• Child maintenance is to be paid for four or more children 

• The paying parent’s income exceeds £650,000 

• The paying parent’s income is largely unearned 

• The paying parent has no income and lives on capital 

If you require advice regarding how much child maintenance you or your former partner should pay, contact Vardags today for a free initial consultation with one of our expert divorce solicitors. 

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