CMS Calculator Explained: How UK Child Maintenance Is Actually Worked Out

Numbers don’t lie, but they do confuse people, and nowhere is that truer than the CMS formula. I’ve lost count of how many times someone’s told me “I heard it’s just 20% of your wage,” which is close enough to be dangerous and wrong enough to cause real arguments. So let’s actually pull the formula apart, piece by piece, with real numbers plugged in, so you can see exactly where a figure comes from instead of taking anyone’s word for it.

How Is Child Maintenance Calculated in the UK?

The CMS runs everything through a fixed formula rather than a judgment call — the same inputs will always produce the same output. It works in this order:

1. Start with the paying parent’s gross weekly income (before tax and National Insurance), pulled directly from HMRC.
2. Deduct pension contributions.
3. Deduct a percentage if the paying parent has other children living in their household.
4. Apply the relevant rate band and percentage based on income.
5. Reduce the result if the child stays overnight with the paying parent regularly.

That’s it. No judge, no negotiation — just a formula applied consistently, which is exactly why it’s called a “calculation” rather than an “assessment” or “ruling.”

What Is the Formula for Calculating Maintenance Cost?

Here are the actual bands currently used:

• Nil rate: Gross weekly income of £7 or less — no maintenance payable.
• Flat rate: £7.01–£100 a week (or certain benefit recipients) — a fixed £7 a week regardless of children.
• Reduced rate: £100.01–£199.99 a week — £7 plus a tapered percentage.
• Basic rate: £200–£800 a week — 12% for 1 child, 16% for 2 children, 19% for 3 or more.
• Basic plus rate: £800.01–£3,000 a week — the first £800 is charged at the basic rate percentages, and anything above that up to £3,000 is charged at a lower rate (9% for 1 child, 12% for 2, 15% for 3+).

Income above £3,000 a week isn’t included in the standard formula at all — beyond that point, the receiving parent generally needs to apply to the family court for a top-up order.

Basic Child Maintenance Rate: A Worked Example

Let’s make this concrete. Say a paying parent earns £41,600 a year gross, no pension contributions, one child, no shared care.

• £41,600 ÷ 52 = £800 a week gross
• That sits right at the top of the Basic Rate band
• One child = 12%
• £800 × 12% = £96 a week

Now add shared care — say the child stays with the paying parent 60 nights a year. That falls into the 52–103 nights band, which gives a 1/7 reduction:

• £96 − (£96 ÷ 7) ≈ £82 a week

That’s the entire logic. No hidden multipliers, no discretion — just income, percentage, and a shared care adjustment.

How Much Maintenance Should a Father Pay Per Child?

This question comes up constantly, and the honest answer is: it depends entirely on income, not a flat “per child” figure. The CMS doesn’t set a fixed £X-per-child rate — it applies a percentage of gross income, so a father earning £25,000 a year and one earning £60,000 will pay very different amounts for the same single child, even under an identical percentage rule. It’s proportional to income by design, not a flat tariff.

That said, using the example above, someone earning around £41,600/year with one child and no shared care lands at roughly £96 a week (£416/month). Someone on £25,000/year would land closer to £47 a week. The percentage is fixed by number of children; the pound amount moves with income.

How Much Is Child Maintenance for 1 Child?

Since the formula scales with income, “how much for one child” only really makes sense as a range:

• Flat rate band: £7/week regardless of income specifics in that band
• Reduced rate band: somewhere between £7 and roughly £24/week
• Basic rate: 12% of gross weekly income, up to £800/week income (so up to £96/week)
• Basic plus: 12% of the first £800, plus 9% of anything above that up to £3,000/week

Use the official CMS calculator on GOV.UK for your specific number — anything else, including this article, is only an illustration of the method.

What Is Not Included in CMS?

This trips a lot of people up. Standard CMS child maintenance covers everyday living costs — food, clothing, housing contribution, basic day-to-day needs. It does not automatically cover:

• Private school fees
• University tuition or costs
• Extracurricular activities (music lessons, clubs, sports kit)
• School trips
• One-off costs like a laptop for school or a birthday gift

These sit outside the CMS formula entirely. If you want a parent to contribute to these, that’s a private negotiation between parents — or, in high-income or high-conflict cases, something the family court can be asked to address separately through additional financial provision.

If I Pay Child Maintenance, Should I Pay for Anything Else?

Legally, no — the CMS calculation is designed to be the full statutory obligation. Practically, though, most families I’ve come across do end up splitting some “extras” informally, especially things like school trips or a share of uniform costs, simply because it keeps the co-parenting relationship functional. But there’s no legal requirement to pay beyond the calculated amount unless you’ve separately agreed to it, or a court has ordered additional provision (which tends to happen mainly in higher-income cases involving things like school fees).

Can Child Maintenance Be Backdated?

No — and this catches people out more than almost anything else. The CMS calculates and collects maintenance from the date your application is accepted, not from the date you separated or the date payments stopped. If there’s been a gap since you last received support, that gap generally isn’t covered retroactively through CMS. This is exactly why advice from most family solicitors is: don’t wait to apply if payments have lapsed. Court-ordered maintenance, by contrast, can sometimes be backdated depending on the circumstances — which is one of the few areas where going through court differs meaningfully from the CMS route.

Child Maintenance Login: Where to Sign In

Once your case is set up, you manage everything through your Child Maintenance Service online account on GOV.UK — the same portal you used to apply. Signing in lets you check payment history, see your Expected Payments Plan, update your details, and message the CMS directly rather than phoning. If you’ve forgotten your login details, the “forgotten password” route on the sign-in page handles it the same way most government accounts do — you’ll need access to the email address the account was registered with.

Child Maintenance Advice for Fathers

A few things I’d genuinely tell any father going through this:

• Income is pulled from HMRC, not self-reported, so there’s little point disputing your own PAYE income — it’s verified automatically. Self-employed income is a different story and can be worth reviewing carefully.
• Shared care genuinely reduces the amount, and it’s worth formally reporting overnight stays if they’ve increased, rather than assuming CMS already knows.
• Paying through “Direct Pay” (private arrangement under a CMS calculation) avoids collection fees that apply if CMS has to collect and pass on the payment for you — currently a surcharge added for the paying parent and a deduction taken from the receiving parent’s payment under “Collect and Pay.”
• If your income changes significantly, request a recalculation rather than waiting for the annual review — the CMS conducts an annual check, but big changes mid-year (redundancy, new job, big pay rise) can usually trigger an earlier review if you report them.

Common Mistakes to Avoid

• Assuming maintenance is a flat “per child” fee. It’s a percentage of income, not a fixed figure.
• Ignoring shared care reporting. If your overnight stays have changed and you don’t report it, you could be overpaying or underpaying without realizing.
• Thinking CMS covers school fees or extracurriculars. It doesn’t — those are separate, private, or court matters.
• Assuming CMS payments will be backdated to when things went wrong. They won’t — apply promptly.
• Not using the official CMS calculator for your actual figure. Third-party calculators (including examples in guides like this one) are illustrations of the method, not your definitive number.
• Missing a mid-year change in income. A pay rise, redundancy, or new job should trigger a fresh look at the calculation, not be left until the annual review.

Final Thoughts

The formula itself is actually more transparent than most people expect once you see it laid out — income, percentage, shared care, done. The confusion usually comes from mixing it up with things it was never designed to cover, like school fees or one-off costs, or expecting it to reach back in time to cover a gap that’s already happened. Once you understand the mechanics, it stops feeling like a mystery and starts feeling like what it actually is: a fairly blunt but predictable formula, applied the same way to everyone.

FAQ

How is child maintenance calculated in the UK?
Using gross weekly income (from HMRC), adjusted for pension contributions and other children in the household, then applying a percentage based on rate band and number of qualifying children, with a further reduction for shared overnight care.

What is the formula for calculating maintenance cost?
Nil rate (£7 or under/week = £0), flat rate (£7/week), reduced rate (tapered), basic rate (12/16/19% for 1/2/3+ children on income £200–£800/week), and basic plus rate (lower percentages on income between £800 and £3,000/week).

What is not included in CMS?
Private school fees, university costs, extracurricular activities, school trips, and one-off expenses — these sit outside the standard formula and are handled privately or through the courts in some cases.

What is the biggest factor in calculating child support?
The paying parent’s gross weekly income is the primary driver of the calculation, more than any other single factor.

Where can I use a child maintenance calculator?
The official CMS calculator is available on GOV.UK and uses your actual reported figures — third-party calculators can illustrate the method but shouldn’t replace it.

If I pay child maintenance, should I pay for anything else?
Not legally, beyond the CMS-calculated amount, unless you’ve privately agreed to extras or a court has ordered additional provision.

How much maintenance should a father pay per child?
It’s proportional to gross income and number of children rather than a flat amount — the same percentage applies regardless of parental gender.

What’s the basic child maintenance rate?
12% of gross weekly income for one child, 16% for two, and 19% for three or more, applied within the £200–£800 weekly income band.

How do I log into Child Maintenance Service?
Through the same GOV.UK portal used to apply — sign in with the email address and password used to set up your account.

How much is child maintenance for 1 child?
It ranges from £7 a week at the flat rate up to roughly £96 a week at the top of the basic rate band, and higher again in the basic plus band — entirely dependent on the paying parent’s income.

What advice is there for fathers dealing with child maintenance?
Report shared care changes promptly, use Direct Pay to avoid collection fees, request a recalculation after major income changes, and don’t dispute PAYE income that’s already HMRC-verified.

Can child maintenance be backdated?
Generally no — CMS payments start from the date of application, not from when payments stopped or the separation occurred, though court-ordered maintenance can sometimes be backdated depending on circumstances.