Child Maintenance Arrears Calculator UK

What this calculator does

DocketMath’s Child Maintenance Arrears Calculator (UK) helps you estimate the arrears amount where child maintenance hasn’t been paid (or has been paid inconsistently) for a period of time.

In practical terms, the calculator is built to let you:

• Model a date range (for example, from a missed-payment start date to an end date)
• Apply the maintenance amount(s) for that range (e.g., a weekly rate)
• Handle payment frequency (weekly, every 2 weeks/fortnightly, monthly) so the expected charges match the way maintenance was assessed
• Output an estimated arrears total you can use to:

  • prepare for discussions with the other parent,
  • understand what you may owe (or be owed), and
  • sanity-check your expectations before you take next steps

This tool is designed for planning and comparison, not an official determination. If you need an exact arrears figure for a formal dispute, enforcement action, or a court process, you should rely on the official calculation method used by the relevant service and/or the specific details shown in the maintenance calculation records.

If you want to run the numbers now, use the tool here: /tools/cms-child-maintenance .

When to use it

Use DocketMath’s arrears calculator when you have enough information to define:

1. the relevant period, and
2. the maintenance amount(s) that applied during that period.

This is especially useful when:

• You’re trying to understand a claim: You’ve been told a particular arrears figure and want to check whether it’s broadly consistent with your payment pattern.
• You’re preparing for a negotiation: You want a working figure before proposing a repayment plan or discussing settlement terms.
• You’re correcting your own records: There may be gaps or you’re unsure whether you covered certain weeks/months.
• You’re dealing with a change: Maintenance may have changed during the time window (for example, after a revision). You can split the period into separate sections and model each section using the correct rate.

What counts as “arrears” for planning purposes

For calculators like this, “arrears” typically means:

expected maintenance for the period − payments actually made for that period

Because this is a calculation aid (not a definitive legal calculation), it’s fine to enter approximate figures if that’s all you have. The key is to then compare scenarios (for example, shifting the start date slightly or using a different frequency) to understand how sensitive the outcome is.

Inputs to gather before you start

Check you have the building blocks:

• ✅ Start date and end date (or “up to today”)
• ✅ The maintenance amount relevant to that time (and the frequency: weekly, fortnightly, monthly)
• ✅ Payment frequency and timing (if known)
• ✅ Partial payments made during the period (if you’re estimating net arrears)

Step-by-step example

Below is a realistic walkthrough using example dates and a weekly maintenance amount. Swap in your own figures.

Scenario: missed weekly payments for a defined period

• Maintenance assessment is £90 per week
• Payments were missed starting 1 March 2026
• The period continues until 30 April 2026
• For simplicity, no payments were made during that window

Step 1: Set your arrears period

• Start date: 1 March 2026
• End date: 30 April 2026

The calculator will convert the date range into an expected number of chargeable periods based on your selected frequency.

Step 2: Enter maintenance amount and frequency

• Maintenance amount: £90
• Frequency: weekly

If your real situation is fortnightly or monthly, select the matching frequency, because the arithmetic changes when converting your date range into expected charges.

Step 3: Add payments made (if any)

• Payments made during the period: £0

If you made partial payments, enter those amounts so the calculator can reduce the expected total accordingly.

Step 4: Review the output

The calculator will produce an estimate based on inputs like:

The exact number depends on how the tool translates your date range into charge periods. If your missed period begins or ends close to a boundary, it can be helpful to run quick variations to see if the total is stable.

Example with partial payments

Now suppose you actually paid £180 during March but missed the rest.

• Your expected maintenance for the period is still based on £90/week
• Enter total payments made for the same date window (for example, £180)

In simple terms:

• Gross expected maintenance − payments already made = estimated arrears balance

Pitfall to avoid: If you enter “payments made” but those payments don’t line up with the same date window you used for the arrears period, you can accidentally double-count weeks (or miss them). Keep the date range and payment entries consistent.

Common scenarios

Arrears won’t look the same in every case. Here are common patterns, and how to model them effectively in DocketMath.

1) Maintenance amount changed during the period

If the weekly/fortnightly/monthly amount changed partway through, don’t average it—split the timeline into separate date ranges:

• Range A: start date → change effective date → amount A
• Range B: day after the effective date → end date → amount B

Then add the arrears totals from each range.

Checklist:

2) The arrears claim starts before you have records

Sometimes you only have records from a later date. You can still use the calculator, but be clear about scope:

• Use the earliest date you can support reliably as your start date
• Treat the output as: “estimated arrears for the recorded period”

You can also test sensitivity:

• run another scenario using a slightly earlier guessed start date and compare results

3) Payments were made irregularly

If payments aren’t uniform, a simple “total paid” figure may still give a reasonable planning estimate, but accuracy improves if you can align payment dates.

Options:

• If you only know totals: enter lump sums that correspond to the date window you selected
• If you have payment history by date: align those dates to your arrears window (or split into segments)

4) Up-to-date arrears (“from start date until today”)

For ongoing arrears:

• set Start date to when you believe the missed period began
• set End date to today (or the latest date you want to estimate up to)

If later payments occur, re-run the tool with an updated end date or add the new payments to refine your estimate.

5) Multiple children or different arrangements

Some cases involve multiple arrangements or separate charges.

Safer approach:

• run the calculator per arrangement/amount (if you can clearly separate them by date and rate)
• then sum the arrears totals

If you can’t clearly map your inputs, use the estimate as planning guidance and avoid assuming it represents an official figure.

6) Relationship changes or custody changes

Even where circumstances change, arrears generally depend on the maintenance obligation that was in force during each period.

If you’re unsure which rate applied when:

• build scenarios using the most reliable amounts/dates you have
• treat the result as an estimate rather than a determination

Warning (gentle): Don’t assume that a later change in circumstances automatically corrects arrears for earlier periods. For planning, it’s best to model the obligations that applied during the relevant timeframe.

Tips for accuracy

You’ll get better estimates when your inputs reflect how the expected charges are being counted.

Use date ranges deliberately

• Choose the earliest date you want included.
• Keep the end date clear and intentional (for example, the day before a new rate starts, or a specific cut-off date for your estimate).
• If you compare scenarios, keep the end date fixed so you’re comparing like with like.

Keep frequency consistent

Entering a weekly amount while selecting monthly frequency (or vice versa) can produce wildly incorrect results.

Double-check:

• weekly vs fortnightly vs monthly
• whether the calculator expects the figure as “per period” (based on your selected frequency) rather than annualised

Split periods when you know the amount changed

When you have evidence of a change:

• ✅ splitting into two (or more) segments is usually more accurate
• ❌ blending into one average can hide differences, especially around boundary dates

Account for partial payments using the same window logic

To reduce mismatches:

• total payments within the exact date window you selected
• ensure payments aren’t counted outside that window
• if you only know totals by month, test how the estimate shifts by adjusting the boundary by a few days

Run boundary checks near the start and end dates

Date-to-period mapping can create “off-by-one” effects. A quick way to gauge estimate quality is to test:

If the total changes substantially with a one-day shift, treat the estimate as less reliable and gather more precise information about effective dates and payment timing

Related reading

• Spreadsheet checks before running interest in United Kingdom — Spreadsheet validation before import
• How to interpret interest results in United Kingdom — What each output means and what moves the result
• Choosing the right interest tool for United Kingdom — How to choose the right calculator