How Child Maintenance Is Calculated in the UK: A Complete Guide

Child maintenance causes more confusion and more arguments between separating parents than almost any other financial question. Both sides often arrive at wildly different numbers, each convinced theirs is correct. Most of the time, the disagreement comes down to a genuine misunderstanding of how the Child Maintenance Service actually works, rather than bad faith on either side.

This guide works through the CMS formula in plain terms, covering how income is assessed, which rate applies, how shared care changes the calculation, and what your options are for making or receiving payments. You can also use our child maintenance calculator to get a figure based on your specific circumstances.

What the Child Maintenance Service is and when it applies

The Child Maintenance Service is the government body responsible for calculating and, where necessary, collecting child maintenance payments. It replaced the old Child Support Agency in 2012, though many people still use the two names interchangeably. CMS applies to children under 16, or under 20 if they are in full-time non-advanced education or approved training.

Parents can reach a private agreement between themselves without involving the CMS at all. These family-based arrangements have no legal standing in the same way, but if both parents agree and stick to it, there is no need for a third party. The CMS only becomes directly involved when one parent applies to use the service, usually because agreement has broken down or was never possible.

There is a £20 application fee to use the CMS, though this is waived if you have experienced domestic abuse or violence. Once you apply, the CMS contacts the other parent, assesses income, and calculates a payment figure based on its statutory formula.

The four CMS rates and how they work

The CMS uses four different rates depending on the paying parent's gross weekly income. It is the paying parent's income that drives the calculation, not the receiving parent's financial situation.

The basic rate is the one most working parents fall into. If your gross weekly income is £700 and you have two children living primarily with the other parent, the calculation is straightforward: 16% of £700 gives you £112 per week. That works out at roughly £484 per month. The percentage does not change as income rises, so the absolute payment grows proportionally with earnings up to the income cap.

Above £3,000 per week gross income, the basic rate formula applies up to that cap and a parent can apply to a family court for an additional top-up if they believe the CMS figure is insufficient given the paying parent's wealth. This is relatively rare in practice and involves litigation, so most families are not affected by it.

The reduced rate applies in the £200 to £400 weekly income band and uses a sliding scale rather than a flat percentage. The formula starts from the flat rate of £7 and adds an increasing fraction as income rises, reaching the full basic rate percentage at the top of the band. The exact figure depends on exactly where income falls within that range, which is why the online calculator is more reliable than doing it by hand for this group.

How the CMS assesses income

Income assessment is where a lot of parents get tripped up. The CMS bases its calculation on gross taxable income, which means income before income tax and National Insurance but after pension contributions. It typically uses the most recent tax year data from HMRC rather than current earnings, so if your income has changed significantly, this can create a gap between what the CMS says you earn and what you actually earn now.

If the paying parent is employed, HMRC data usually reflects their salary accurately. If they are self-employed, the CMS uses their reported profits from the last completed tax return. This is where disputes often arise, because self-employed income can vary year on year and business expenses reduce the taxable profit figure. The CMS can request additional financial information if it believes declared income does not reflect actual earnings.

The CMS also has the power to look at unearned income such as rental income, dividends, and investment returns if these total more than £2,500 per year. If a parent has restructured their affairs to minimise the income that shows up on their tax return, the CMS can refer the case for a variation, which allows it to assess income on a different basis. Broadly speaking, the more financially complex a paying parent's situation, the more important it becomes to get professional advice.

How shared care reduces the payment

If the child stays overnight with the paying parent on a regular basis, the CMS reduces the weekly payment to reflect this. The reduction works in sevenths, corresponding to how many nights per week on average the child stays with the paying parent.

So if you pay £112 per week as calculated above, and the child stays with you two nights per week on average (104 to 155 nights per year), the two sevenths reduction brings your payment down to roughly £80 per week. This makes the overnight count genuinely significant for many families, and it also means that disputes about who the child actually sleeps with can have a real financial dimension.

The CMS looks at average overnight stays across the year. If care patterns change, either parent can ask for a review, and the CMS will reassess based on the new arrangements. This is worth doing if there has been a genuine increase in shared care rather than a temporary change.

Private agreements versus CMS involvement

Many parents prefer to make their own arrangements because they are faster, cheaper, and more flexible than going through the CMS. A private agreement can be for any amount the two parents agree on, and you can set it up however suits the family, weekly, monthly, annually, or as a one-off lump sum for specific costs.

The limitation of a private arrangement is that it has no automatic enforcement mechanism. If the paying parent stops paying or pays less than agreed, the receiving parent cannot ask the CMS to enforce a private agreement. They would need to start a fresh CMS application from scratch. Some parents make a private arrangement and then verify it roughly matches what the CMS formula would produce, just to have a shared reference point. Running the numbers through the CMS calculator before agreeing on a figure can help both sides feel the arrangement is fair.

For parents who have previously tried private arrangements but found them unreliable, the CMS's collect and pay service exists specifically to handle payment collection. The paying parent pays the CMS directly and the CMS pays out to the receiving parent. This is more expensive for everyone — the paying parent has 20% added to their assessment and the receiving parent has 4% deducted — but it does mean the CMS can pursue enforcement action if payments are missed.

When a paying parent doesn't pay

The CMS has substantial enforcement powers for parents who refuse to pay or fall into arrears. These include deducting payments directly from wages, known as a deduction from earnings order, which is sent to the employer and operates like a court order. The employer must comply.

Beyond that, the CMS can freeze and seize funds from bank accounts, apply for a charging order on property, require the payment parent to surrender their passport, or in serious cases apply to court for a curfew or even a prison sentence. These more serious steps are for persistent and deliberate non-payment rather than short-term difficulties, and the CMS will typically try the deduction from earnings route first.

If you are struggling to pay because your circumstances have genuinely changed, the right approach is to contact the CMS proactively and request a review of your assessment. Ignoring the payments and hoping the problem goes away tends to make things significantly worse. Arrears attract enforcement action and can grow quickly.

Requesting a review if circumstances change

Both parents can request an annual review of the child maintenance calculation. Outside of the annual review, a review is triggered automatically if income changes by more than 25% compared to the figure used in the current assessment. So if your salary drops substantially due to redundancy or a career change, you can request a reassessment based on your new income without waiting for the annual review cycle.

Variations can also be requested in specific circumstances. A paying parent can apply for a variation if they have high travel costs to see the child, or significant debts accrued before the child's birth that relate to the family home. A receiving parent can request a variation if they believe the paying parent has assets or income not reflected in their HMRC-reported figures, or if the paying parent has transferred assets to avoid paying.

The child maintenance system is not designed to be punitive, though it can feel that way when you are on either side of it. The CMS formula is meant to reflect what a child would have received had the family remained together, broadly speaking. Where both parents are cooperative and the paying parent's income is straightforward, the system works reasonably well. Where there is conflict or financial complexity, getting legal or financial advice early tends to save time and money compared to letting disputes escalate through the CMS's formal process.