Which Child Support Calculator Should Australian Parents Use?

We tested the same scenario across the four main child support calculators used in Australia. Only one matched Services Australia exactly.

Australian parents often rely on online child support calculators to understand what they may be required to pay or receive. These tools are used for budgeting, planning, and testing “what-if” scenarios before or between official assessments.

However, while all calculators claim to apply the Australian child support formula, they do not all implement it correctly.

To assess accuracy, we applied the same real-world scenario to the four main calculators commonly used in Australia.

1. Services Australia Child Support Estimator (official)
2. ChildSupportAustralia.com Calculator
3. Unified Lawyers Child Support Calculator
4. Custody X Change Australian Calculator

The results were not the same.

This article documents what happens when the legislated formula is applied precisely, and where other calculators introduce structural or mathematical errors.

The Test Scenario (Identical Across All Calculators)

The following inputs were used in every calculator.

• Parent A taxable income: $100,000
• Parent B taxable income: $80,000
• Two children:
  • one aged 0–12
  • one aged 13–17
• Parent A care: 130 nights per year (5 nights per fortnight)
• No relevant dependent children
• No multi-case arrangements

This is a common, uncomplicated scenario that falls squarely within the standard administrative formula.

Services Australia Child Support Estimator (Reference Result)

The Services Australia estimator is the reference point, as it implements the Child Support (Assessment) Act 1989 directly.

Result

• Annual child support payable: $9,974
• Monthly: $831

The estimator provides a detailed step-by-step breakdown, including child support income, combined income, income percentages, care percentages, cost percentages, costs of children, and per-child allocation.

Despite its complexity, this output represents the authoritative application of the legislation.

ChildSupportAustralia.com Calculator (Exact Match)

The ChildSupportAustralia.com calculator was designed to apply the legislated formula directly, without replicating Services Australia’s internal administrative workflow.

Result

• Annual child support payable: $9,974
• Monthly: $831

This result identically matches the Services Australia estimator for the same scenario.

Why the match is exact

The calculator applies the official Costs of Children Table, uses the correct income band base amounts, applies marginal rates only above the relevant threshold, handles care percentages and cost percentages correctly, and allocates costs per child while rounding at the same stage as Services Australia.

During development, we tested edge cases involving mixed-age children, shared care thresholds, rounding at band boundaries, and per-child allocation effects. This matters because small rounding errors compound later in the formula and can change the final assessment.

Unified Lawyers Child Support Calculator (Inaccurate)

The Unified Lawyers calculator produces a different result for the same inputs.

Result

• Annual child support payable: $9,630
• Monthly: $803

This is lower than the legislated outcome.

Identified issues

• The calculator does not reproduce the official cost-of-children calculation.
• Internal rounding differs from the statutory method.
• Results do not align with Services Australia for identical inputs.
• Each scenario change requires pressing “submit”, making it hard to test different settings quickly.

In addition to the numerical discrepancy, the workflow makes it harder to identify where the calculation diverges from the legislation.

For a financial estimate, repeatability and transparency matter. This calculator does not provide either.

Custody X Change Australian Calculator (Structurally Incomplete)

The Custody X Change calculator produces a substantially different outcome.

Result

• Annual child support payable: approximately $6,720
• Monthly: approximately $560

This is around 30–35% lower than the legislated result.

Why the result is unreliable

The Australian version of this calculator does not collect several mandatory inputs required by Australian law.

• It does not ask for the age of each child.
• It therefore cannot apply the correct Costs of Children Table.
• It relies on monthly income inputs rather than annual taxable income.
• It applies an internal model that is not the Australian statutory formula.

Under Australian law, the cost of a child aged 0–12 is materially different from the cost of a child aged 13–17. A calculator that ignores child ages cannot produce a legally consistent estimate.

This is not a rounding issue. It is a missing-inputs problem.

Why These Differences Matter

Child support calculations are cumulative. Errors introduced early in the process affect income percentages, cost percentages, costs of children, and final payable amounts.

Even a small structural shortcut can result in hundreds or thousands of dollars per year of difference.

Parents often rely on calculators to plan budgets, assess affordability, negotiate private arrangements, and decide whether to seek a reassessment. A calculator that does not follow the legislated method can mislead parents into thinking an amount is fair or correct when it is not.

What a Reliable Australian Child Support Calculator Must Do

To align with the Child Support (Assessment) Act 1989, a calculator must do the following.

• Use annual taxable income.
• Apply the self-support amount correctly.
• Use the official Costs of Children Tables.
• Distinguish between children aged 0–12 and 13–17.
• Calculate care and cost percentages accurately.
• Allocate costs per child and round at the correct stage.
• Apply income bands using base plus marginal, not reconstructed proxies.

Omitting any of these steps produces a different result.

Conclusion

We applied the same scenario to the four most commonly used child support calculators in Australia.

• Services Australia provides the authoritative benchmark.
• ChildSupportAustralia.com is the only independent calculator that identically matches the government result for this scenario.
• Unified Lawyers produces a different figure and makes scenario testing cumbersome.
• Custody X Change omits mandatory inputs and produces outcomes inconsistent with Australian law.

For parents seeking clarity and accuracy, methodology matters more than presentation. A calculator should reflect the law as written, not an approximation of it.

Why We Conducted This Comparison

ChildSupportAustralia.com has operated a child support calculator continuously since 2017. Over that time, we have responded to thousands of questions from parents trying to understand how the Australian child support formula applies to their circumstances.

Through that experience, we are well aware that the legislated calculation is complex. Small implementation choices around income bands, cost tables, care percentages, and rounding stages can materially affect the final result.

We conducted this comparison because of ongoing concerns about the accuracy of third-party calculators used by Australian parents. The purpose was not to criticise competing tools, but to verify whether commonly used calculators reproduce the outcomes generated by Services Australia when the same inputs are applied.

This testing also serves an internal purpose. Because we are independent of Services Australia, it is essential that our calculator continues to reflect the law as written and produces results that align with the official estimator for standard scenarios.

Parents rely on child support estimates to make real financial decisions. In that context, accuracy is not a feature or a preference. It is a responsibility.

Disclaimer:
This comparison is based on publicly available calculators and the legislated Australian child support formula. It is not legal advice and does not replace a formal assessment issued by Services Australia. Temporary discrepencies in estimator results can occur while annually indexed figures are being updated.