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

judge Mostyn’s formula for ‘top-up’ orders

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’.

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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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

TABLE 2: FORMULA ADJUSTED		A CSSP per child no shared care			B CSSP per child: ¹/₇^thshared care			C CSSP per child ²/₇^ths shared care			D CSSP per child ³/₇^ths shared care			E CSSP per child equal shared care
	No. of children	1	2	3	1	2	3	1	2	3	1	2	3	1	2	3
Amount of E	156,000	15,300	10,200	8,400	13,100	8,700	7,200	10,900	7,300	6,000	8,700	5,800	4,800	7,300	4,400	3,100
	175,000	15,700	10,800	8,900	13,500	9,300	7,600	11,200	7,700	6,400	9,000	6,200	5,100	7,500	4,700	3,400
	195,000	16,200	11,400	9,500	13,900	9,800	8,100	11,600	8,100	6,800	9,300	6,500	5,400	7,700	5,000	3,700
	215,000	16,700	12,000	10,100	14,300	10,300	8,700	11,900	8,600	7,200	9,500	6,900	5,800	8,000	5,300	4,000
	235,000	17,200	12,600	10,700	14,700	10,800	9,200	12,300	9,000	7,600	9,800	7,200	6,100	8,200	5,600	4,300
	255,000	17,700	13,200	11,300	15,200	11,300	9,700	12,600	9,400	8,100	10,100	7,500	6,500	8,500	5,900	4,600
	275,000	18,100	13,800	11,900	15,500	11,800	10,200	12,900	9,900	8,500	10,300	7,900	6,800	8,700	6,200	4,900
	295,000	18,600	14,400	12,500	15,900	12,300	10,700	13,300	10,300	8,900	10,600	8,200	7,100	8,900	6,500	5,200
	315,000	19,100	15,000	13,100	16,400	12,900	11,200	13,600	10,700	9,400	10,900	8,600	7,500	9,200	6,800	5,500
	335,000	19,600	15,600	13,700	16,800	13,400	11,700	14,000	11,100	9,800	11,200	8,900	7,800	9,400	7,100	5,800
	355,000	20,100	16,200	14,300	17,200	13,900	12,300	14,400	11,600	10,200	11,500	9,300	8,200	9,700	7,400	6,100
	375,000	20,500	16,800	14,900	17,600	14,400	12,800	14,600	12,000	10,600	11,700	9,600	8,500	9,900	7,700	6,400
	395,000	21,000	17,400	15,500	18,000	14,900	13,300	15,000	12,400	11,100	12,000	9,900	8,900	10,100	8,000	6,700
	415,000	21,500	18,000	16,100	18,400	15,400	13,800	15,400	12,900	11,500	12,300	10,300	9,200	10,400	8,300	7,000
	435,000	22,000	18,600	16,700	18,900	15,900	14,300	15,700	13,300	11,900	12,600	10,600	9,500	10,600	8,600	7,300
	455,000	22,500	19,200	17,300	19,300	16,500	14,800	16,100	13,700	12,400	12,900	11,000	9,900	10,900	8,900	7,600
	475,000	22,900	19,800	17,900	19,600	17,000	15,300	16,400	14,100	12,800	13,100	11,300	10,200	11,100	9,200	7,900
	495,000	23,400	20,400	18,500	20,100	17,500	15,900	16,700	14,600	13,200	13,400	11,700	10,600	11,300	9,500	8,200
	515,000	23,900	21,000	19,100	20,500	18,000	16,400	17,100	15,000	13,600	13,700	12,000	10,900	11,600	9,800	8,500
	535,000	24,400	21,600	19,700	20,900	18,500	16,900	17,400	15,400	14,100	13,900	12,300	11,300	11,800	10,100	8,800
	555,000	24,900	22,200	20,300	21,300	19,000	17,400	17,800	15,900	14,500	14,200	12,700	11,600	12,100	10,400	9,100
	575,000	25,300	22,800	20,900	21,700	19,500	17,900	18,100	16,300	14,900	14,500	13,000	11,900	12,300	10,700	9,400
	595,000	25,800	23,400	21,500	22,100	20,100	18,400	18,400	16,700	15,400	14,700	13,400	12,300	12,500	11,000	9,700
	615,000	26,300	24,000	22,100	22,500	20,600	18,900	18,800	17,100	15,800	15,000	13,700	12,600	12,800	11,300	10,000
	635,000	26,800	24,600	22,700	23,000	21,100	19,500	19,100	17,600	16,200	15,300	14,100	13,000	13,000	11,600	10,300
	650,000	27,100	25,000	23,200	23,200	21,400	19,900	19,400	17,900	16,600	15,500	14,300	13,300	13,200	11,800	10,500

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

judge Mostyn’s formula for ‘top-up’ orders

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