CSA Calculator UK: How to Work Out Child Maintenance Payments in 2025

If you search for a CSA calculator or CMS calculator, you are usually trying to answer one question: how much child maintenance should I pay or receive? For most UK parents, the answer follows a statutory formula based on the paying parent's gross weekly income, the number of children, and how many nights the children stay with the paying parent each year.

A parent earning £45,000 gross per year with one child and no shared care would typically pay around £104 per week under the basic CMS rate. The same parent with two children would pay roughly £138 per week. These figures change if income falls into a different band, if shared care applies, or if the paying parent has other children living with them.

This guide explains how a child maintenance calculator works, what inputs matter, and how to interpret the result before you apply to the Child Maintenance Service or negotiate a private arrangement. Use our child maintenance calculator to model your own figures in minutes.

CSA, CMS and child maintenance: what the names mean

Many people still say CSA calculator when they mean the current Child Maintenance Service system. The Child Support Agency was replaced by the CMS in 2012, but the underlying idea is the same: a government formula calculates how much the non-resident parent should contribute toward the costs of raising a child.

The CMS applies when parents cannot agree a family-based arrangement, or when one parent applies to use the service. The calculation is based on the paying parent's income, not the receiving parent's financial needs or outgoings. That surprises some people, but it is how the statutory scheme is designed.

For a full overview of how the system fits together, see our guide on how child maintenance works in the UK. For a deeper look at the 2025 formula and rate bands, read how CMS child maintenance is calculated in 2025.

What a child maintenance calculator needs from you

A reliable CMS calculator asks for a small set of inputs. The paying parent's gross annual or weekly income is the starting point. You also need the number of children the maintenance is for, whether the paying parent has other children living in their household, and how many nights per year each child stays overnight with the paying parent.

Gross income means taxable income before income tax and National Insurance, but after certain pension contributions. The CMS typically uses HMRC data from the most recent tax year, so your calculator result may differ from a live CMS assessment if your earnings have changed significantly since then.

Shared care is often the input that changes the result most dramatically. A parent who has the children two nights per week on average can see their weekly payment reduced by two sevenths. Over a year, that can amount to thousands of pounds in difference between what each parent expected and what the formula produces.

The four CMS income bands explained

The CMS calculator assigns the paying parent to one of four rates depending on gross weekly income. Most working parents fall into the basic rate, where a fixed percentage of income is applied based on the number of children.

Below £100 per week gross, the nil rate applies and no maintenance is due under the formula. Between £100 and £200 per week, the flat rate of £7 per week applies in most cases. Between £200 and £400 per week, the reduced rate uses a tapered calculation that bridges the flat rate and the full basic rate percentage.

Above £400 per week gross, the basic rate applies: 12% for one child, 16% for two children, and 19% for three or more. These percentages apply to the full weekly income figure up to the CMS cap of £3,000 per week. Income above that cap is ignored unless a court orders a top-up, which is uncommon.

How other children in the household affect the rate

If the paying parent has other children living with them, including stepchildren in some circumstances, the CMS reduces the percentage applied. For one qualifying child in the household, the rate drops from 12% to 11% for one maintenance child, from 16% to 14.5% for two, and from 19% to 16.5% for three or more. With two or more qualifying children in the household, the percentages fall further.

This prevents a paying parent from being assessed at the full rate when they are already supporting children in their own home. It is one of the most overlooked factors in informal calculations, and getting it wrong is a common reason parents argue about fairness.

Worked example: basic rate with no shared care

Consider a paying parent with gross income of £52,000 per year. That is £1,000 per week gross, well above the basic rate threshold. They have two children who live primarily with the other parent, no other children in their household, and no shared overnight care.

The basic rate for two children is 16%. Sixteen percent of £1,000 is £160 per week, or roughly £693 per month. The receiving parent would get this figure if the CMS collects and pays on their behalf, minus a 4% collection fee. The paying parent would pay 20% on top if using collect and pay, making the total cost £192 per week.

Run your own numbers through the child maintenance calculator rather than relying on rounded monthly estimates. Weekly figures are how the CMS assesses and enforces payments, and small rounding differences add up over a year.

Shared care: how overnight stays change the payment

When children stay overnight with the paying parent on a regular basis, the CMS reduces the maintenance payment to reflect the costs the paying parent bears directly. The reduction works in sevenths, tied to how many nights per week on average the child stays over the course of a year.

Returning to the £160 per week example, if the children stay two nights per week on average with the paying parent, the two sevenths reduction brings the payment down to roughly £114 per week. That is a reduction of £46 per week, or more than £2,300 per year. Both parents should understand this before agreeing a private figure that ignores shared care entirely.

The CMS counts overnights, not contact hours. A child who spends Sunday afternoon with the paying parent but sleeps at the other parent's home does not count toward shared care. Keeping a simple calendar record of overnight stays can help if either parent later disputes the arrangement.

When calculator results differ from the CMS assessment

A CSA calculator gives an estimate based on the information you enter. The official CMS assessment may differ for several reasons. The CMS uses HMRC income data, which can lag behind your current salary by a year or more. If you have had a pay rise, redundancy, or period of self-employment, the assessed income may not match what you earn today.

Self-employed parents are assessed on reported profits from tax returns, not turnover. A business owner who reinvests heavily or claims substantial expenses may have a lower assessed income than their gross business receipts suggest. The CMS can investigate if it believes income is being understated.

Benefits affect which rate applies. A paying parent on certain qualifying benefits may pay the flat rate of £7 per week regardless of other income. Universal Credit, income-based Jobseeker's Allowance, and Income Support are among the benefits that trigger the flat rate in most cases.

Private arrangements versus using the CMS

Many parents use a calculator to agree a private family-based arrangement without involving the CMS at all. This is legal, flexible, and avoids the £20 application fee and potential collection charges. You can agree any amount, paid weekly, monthly, or as contributions toward specific costs like school fees or activities.

The limitation is enforcement. If a private arrangement breaks down, the receiving parent cannot ask the CMS to enforce what was agreed privately. They would need to make a fresh CMS application. Using a calculator to benchmark a private figure against the statutory formula gives both sides a neutral reference point and reduces the risk of one parent feeling short-changed.

If you apply to the CMS, you can choose direct pay, where the paying parent transfers money to the receiving parent, or collect and pay, where the CMS handles collection. Collect and pay adds 20% to the paying parent's bill and deducts 4% from what the receiving parent receives. Most parents who can communicate reasonably well choose direct pay to avoid these fees.

Common mistakes when using a child maintenance calculator

The most frequent error is using net income instead of gross. The CMS formula works on gross taxable income, so entering your take-home pay will produce a figure that is far too low. Another common mistake is forgetting to account for shared care nights, or counting daytime contact as overnight stays.

Parents sometimes include the receiving parent's income in the calculation. The CMS formula does not consider what the resident parent earns. Maintenance is calculated solely from the paying parent's income, adjusted for shared care and other children in their household.

Another pitfall is assuming the calculator result is fixed forever. Either parent can ask the CMS to review an assessment if income changes by 25% or more, if shared care arrangements change, or if other relevant circumstances shift. Running the calculator again after a significant life change is sensible before committing to a new private agreement.

What the CMS does not cover

Child maintenance under the CMS formula is intended to cover everyday living costs: food, clothing, housing, and general upbringing. It does not include school fees or large one-off expenses unless parents agree separately.

Step-by-step: using the calculator before you apply

Start by gathering the paying parent's most recent payslip or self-assessment return to confirm gross weekly income. Count the number of children the maintenance is for and note any other children living with the paying parent. Calculate average overnight stays over the past 12 months, or estimate based on the current arrangement.

Enter these figures into the child maintenance calculator and note the weekly and monthly results. Compare the figure to any existing private arrangement. If you are within 10% of the statutory amount, many parents consider that a fair baseline for negotiation.

If you decide to apply to the CMS, the calculated figure gives you a realistic expectation of the outcome. The application costs £20 unless you qualify for a fee waiver. The CMS will contact the other parent, verify income with HMRC, and issue an assessment, usually within a few weeks for straightforward cases.

For most UK parents, a CSA calculator or CMS calculator provides a clear, impartial starting point. Understanding the formula, entering accurate inputs, and accounting for shared care puts you in a much stronger position whether you arrange maintenance privately or through the Child Maintenance Service.