Child Maintenance Calculator UK: How to Work Out What You’ll Pay

Let’s do this properly, together, with an actual pencil-and-paper approach rather than just handing you a calculator link and wishing you luck. Online calculators are useful, sure, but they can feel like a black box — you put a number in, a different number comes out, and you’re none the wiser about why. I’d rather walk you through the actual formula so you understand it, not just trust it.

Grab your gross annual income (that’s before tax, not your take-home pay), and let’s work through this step by step.

Step 1: How Is UK Child Maintenance Calculated? (The Big Picture First)

Before the maths, here’s the shape of the whole process, so you know where we’re headed:

1. Work out gross weekly income
2. Deduct pension contributions, if any
3. Adjust for other children living with the paying parent, if applicable
4. Match that adjusted figure to the correct rate band
5. Apply the percentage for the number of children being calculated for
6. Reduce the result based on shared care overnight stays

Six steps. That’s genuinely it. Let’s go through each one with real numbers.

Step 2: Find Your Gross Weekly Income

Take your gross annual salary and divide by 52. So if you earn £32,000 a year:

£32,000 ÷ 52 = £615.38 per week

The Child Maintenance Service usually pulls this figure directly from HMRC records for the most recent available tax year, so it’s not something you’re estimating from a payslip — it’s based on official data. But doing this calculation yourself first means you can sanity-check whatever figure the CMS eventually comes back with.

Step 3: Deduct Pension Contributions

If you’re contributing to a workplace or personal pension, that amount is deducted from your gross income before the maintenance percentage is applied. So if our example earner puts 5% into a pension:

£615.38 − (5% of £615.38, which is £30.77) = £584.61 adjusted weekly income

This is a genuinely underused detail — increasing pension contributions can legitimately reduce the assessable income figure, though obviously that trades off against your own take-home pay, so it’s not a decision to make purely for this reason.

Step 4: Adjust for Other Children in Your Household

If you have other children living with you (including stepchildren, in some cases), your assessable income is reduced by a percentage before the maintenance calculation is applied — generally somewhere in the 11–16% range, depending on how many other children there are. If this applies to you, factor it in here before moving to the next step.

Step 5: Match Your Income to a Rate Band

Now compare your adjusted weekly figure against the current bands:

• Nil rate — below £7/week (or specific exempt circumstances)
• Flat rate — £7–£100/week: a fixed £7/week regardless of number of children
• Reduced rate — £100.01–£199.99/week: a blended calculation
• Basic rate — £200–£800/week: percentage-based (most people land here)
• Basic Plus rate — £800–£3,000/week: lower percentage, applied only to income above £800

Our example earner, at £584.61/week, sits comfortably in the Basic Rate band.

Step 6: Apply the Percentage

For the Basic Rate band, the percentages are:

• 12% for one child
• 16% for two children
• 19% for three or more children

Let’s say we’re calculating for one child:

£584.61 × 12% = £70.15 per week

That’s roughly £304 per month, before any shared care adjustment.

Step 7: Adjust for Shared Care

If the child stays overnight with the paying parent regularly, the amount reduces based on nights per year:

• 52–103 nights → reduce by 1/7
• 104–155 nights → reduce by 2/7
• 156–174 nights → reduce by 3/7
• 175+ nights (broadly equal care) → reduce by 50%, plus a further small adjustment

If our example parent has the child roughly one night a week (52 nights/year), the reduction is 1/7:

£70.15 × (1 − 1/7) = £70.15 × 6/7 = £60.13 per week, or roughly £260 per month

And that’s your actual worked figure, step by step, no calculator black box required.

What Is the Formula for Calculating Maintenance Cost? (The Short Version)

If you want the whole thing compressed into a single formula for a one-child, Basic Rate case:

(Gross weekly income − pension contributions) × 12% × shared care reduction factor = weekly maintenance

Swap in 16% or 19% for two or three-plus children, and adjust the percentage to the Basic Plus rates if your income sits above £800/week.

What Is the Biggest Factor in Calculating Child Support?

Hands down, it’s gross weekly income. Everything else in the formula — shared care, other children, pension deductions — adjusts that base figure, but income sets the scale of the whole calculation. A small change in reported income shifts the final number more than almost any other variable in the formula.

How to Calculate Maintenance for a Wife? (Spousal Maintenance Is Different)

Worth flagging clearly, because people often conflate the two: spousal maintenance and child maintenance are calculated completely differently in the UK.

Child maintenance follows the fixed CMS formula we just walked through. Spousal maintenance has no fixed formula at all. Instead, courts consider a range of factors under Section 25 of the Matrimonial Causes Act 1973, including:

• Each spouse’s income, earning capacity, and financial needs
• The standard of living enjoyed during the marriage
• The age of each party and the length of the marriage
• Contributions each party made to the family, including non-financial contributions like childcare
• Any physical or mental disability affecting either party

Because there’s no formula to plug numbers into, spousal maintenance amounts are typically negotiated between parties (often through mediation or solicitors) or decided by a court based on the specific circumstances. If you’re trying to work out a spousal maintenance figure, this genuinely isn’t a calculator job — it needs proper legal advice tailored to your situation.

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

Quick but important point: CMS child maintenance is meant to cover day-to-day living costs — food, general upkeep, everyday essentials. It typically does not automatically cover:

• School uniforms or trips
• Childcare costs
• Non-NHS medical or dental costs
• Large one-off expenses

These usually need a separate agreement between parents. If you can’t agree on who covers what beyond the standard maintenance figure, that’s a good candidate for mediation rather than ongoing back-and-forth arguments.

What Age Do You Stop Paying Child Maintenance?

Generally, child maintenance stops when your child turns 16, or continues until 20 if they remain in full-time, non-advanced approved education (A-levels, T-levels, NVQs up to Level 3, and similar). It does not automatically extend to cover university. Importantly, this doesn’t happen automatically through a private agreement — if you’re paying through the CMS, they’ll typically confirm your child’s education status around age 16; if you have a private arrangement, you need to actively review and agree the end date with the other parent.

Practical Details: Applying, Login, and Contact

• Apply for child maintenance: through child-maintenance.service.gov.uk, or by calling 0800 988 0988 for a new application
• Child Maintenance login: manage an existing case, view payment history, and report changes through the same online service
• Child maintenance number (existing cases): 0800 171 2345, Monday–Friday, 8am–6pm

Common Mistakes to Avoid

• Using take-home pay instead of gross income in your own calculations — this produces a lower figure than the CMS will actually use.
• Forgetting to factor in shared care nights when estimating your figure, since this can meaningfully change the outcome.
• Assuming child maintenance covers extras like school trips or childcare without a separate agreement.
• Confusing child maintenance with spousal maintenance. They use entirely different calculation methods.
• Assuming payments stop automatically at 16 without confirming your child’s education status with the CMS or the other parent.

Final Thoughts

Working through the actual formula, rather than just trusting a number a website spits out, genuinely changes how confident you feel going into a conversation about maintenance — whether you’re the one paying or receiving it. Numbers you understand are numbers you can actually discuss calmly. Numbers that feel like a mystery just breed suspicion on both sides.

FAQ

How is UK child maintenance calculated?
Based on the paying parent’s gross weekly income, adjusted for pension contributions and other children, then matched to a rate band and percentage, with a further reduction for shared care nights.

What is the formula for calculating maintenance cost?
(Gross weekly income − pension contributions) × the relevant percentage (12%, 16%, or 19% at Basic Rate) × any shared care reduction factor.

What is the biggest factor in calculating child support?
The paying parent’s gross weekly income — it sets the scale that every other adjustment works from.

How to calculate maintenance for a wife?
Spousal maintenance has no fixed formula; UK courts weigh factors like income, needs, standard of living, and marriage length under the Matrimonial Causes Act 1973, rather than applying a set percentage.