Child Maintenance Service 2026: How the CMS Calculates What You Pay

If you share parental responsibility for a child who does not live with you full time, the Child Maintenance Service (CMS) may be involved in calculating what you pay or receive. The CMS formula is based on gross income rather than take-home pay, which surprises many parents — especially when Income Tax and National Insurance can take a significant slice of earnings before a penny reaches the bank. This guide explains exactly how the CMS works through its calculation in 2026/27.

What Is the Child Maintenance Service?

The CMS is the government body that calculates, collects, and enforces child maintenance in England, Wales, and Scotland. It replaced the Child Support Agency (CSA) and operates under the Department for Work and Pensions. Parents can arrange maintenance privately between themselves, but if they cannot agree, either parent can apply to the CMS.

There is a £20 application fee to use the CMS (waived for victims of domestic abuse and those under 19). Once the CMS is involved, it calculates a statutory maintenance figure that both parents must follow.

Step 1 — Establishing Gross Income

The starting point for every CMS calculation is the paying parent's gross annual income as reported to HMRC. For most employed parents this means total earnings before Income Tax or National Insurance are deducted. The CMS then divides this figure by 52 to arrive at a gross weekly income.

For 2026/27 the main income bands are:

• Below £100 per week — reduced rate applies (see below)
• £100 to £3,000 per week — standard rate percentages apply
• Above £3,000 per week — the standard rate is capped at £3,000/week

The CMS requests income data directly from HMRC so it does not rely on the paying parent's self-declaration for the initial figure. However, if income has changed by 25% or more since the HMRC data was collected, either parent can request a variation.

Step 2 — Permitted Deductions Before the Rate Is Applied

Gross income is not always taken at face value. The CMS allows certain deductions:

• Gross pension contributions — amounts paid into a registered pension scheme can be subtracted. For 2026/27 the pension annual allowance is £60,000, so high earners making large pension contributions can significantly reduce the income figure.
• Other children living with the paying parent — if the paying parent lives with other children (biological or adopted), a percentage reduction is applied to their gross income before child maintenance rates are calculated.

For pension deductions, the relevant figure is the gross contribution — not the net amount paid after basic-rate tax relief. So a £10,000 gross contribution reduces CMS income by £10,000, not £8,000.

Step 3 — Applying the Percentage Rate

Once adjusted gross weekly income is established, the CMS applies a straightforward percentage:

Example: A paying parent earns £45,000 per year gross. Gross weekly income = £45,000 ÷ 52 = £865.38. For one child: £865.38 × 12% = £103.85 per week.

Note that at £45,000, this parent pays Income Tax at 20% on income above £12,570 and Employee NI at 8% on earnings between £12,570 and £50,270. Their actual take-home pay is substantially less than £45,000 — but the CMS does not account for this.

Step 4 — Adjusting for Overnight Stays

If the paying parent has overnight stays with the child, the weekly maintenance figure is reduced to reflect shared care:

Example continued: If the paying parent in the example above has the child to stay for 80 nights per year (1/7 reduction):

• Weekly amount before reduction: £103.85
• Reduction: £103.85 ÷ 7 = £14.84
• Adjusted weekly payment: £88.01

If there are multiple children and they spend different numbers of nights with the paying parent, the CMS applies the reduction separately for each child.

Special Rate Cases

Not all parents fall into the standard rate band. The CMS uses different rates at the extremes of income:

Flat rate — £7 per week Parents whose gross weekly income is less than £7, or who receive certain benefits, pay a flat rate of £7 per week. Benefits that trigger the flat rate include Universal Credit, Income Support, income-based Jobseeker's Allowance, income-related Employment and Support Allowance, and Pension Credit.

Reduced rate — £7 to £100 per week income For gross weekly income between £7 and £100, the CMS applies a sliding reduced rate that transitions between the flat rate and the standard rate.

Nil rate Certain groups pay nothing — for example, full-time students and those in prison.

How Income Is Shared When There Are Multiple Qualifying Children From Different Relationships

Where a paying parent has children with more than one receiving parent, the CMS calculation becomes more complex. The total percentage for all children is calculated first (using the combined rate for the total number of children), and then the total is split proportionally between each receiving parent.

Example: A paying parent has two children with Parent A and one child with Parent B. Gross weekly income is £700.

• Total for three children: 19% × £700 = £133 per week
• Two children (Parent A) receive: 2/3 × £133 = £88.67
• One child (Parent B) receives: 1/3 × £133 = £44.33

Collection Fees and Direct Pay vs Collect and Pay

Once the CMS has calculated the maintenance figure, parents can choose how to handle payment:

Direct Pay — Parents arrange payment between themselves. There is no CMS fee for this arrangement. If the paying parent fails to pay, the case can be moved to Collect and Pay.

Collect and Pay — The CMS collects payment from the paying parent and passes it to the receiving parent. This service carries fees:

• Paying parent: +20% added to the maintenance figure
• Receiving parent: -4% deducted from the maintenance received

On a £100/week maintenance liability, Collect and Pay costs the paying parent £120 and the receiving parent receives £96.

Varying the CMS Calculation

Either parent can apply for a variation to adjust the standard calculation. Common reasons include:

• Additional income not captured by HMRC records, such as income from assets, property rental, or unearned income above £2,500 per year
• High pension contributions that artificially reduce declared income
• Special expenses such as long-distance travel costs to see the child (reducing payments) or unusually high debts related to the relationship (in limited circumstances)

The CMS has discretion over whether to accept a variation request. Variations can increase or decrease the maintenance figure.

Keeping the Calculation in Perspective

The CMS formula uses gross income, which means the effective percentage of take-home pay that child maintenance represents is higher than the headline rates suggest. For a parent earning £45,000 in 2026/27, take-home pay after Income Tax and Class 1 NI is approximately £33,900 per year. Maintenance of £103.85 per week (£5,400 per year) represents around 16% of actual net income — not 12%.

Understanding this distinction matters for budgeting. Use the take-home pay calculator to see exactly what your net position looks like, and the pension calculator to model whether increasing contributions could affect your CMS calculation.