The Guides to Social Policy Law is a collection of publications designed to assist decision makers administering social policy law. The information contained in this publication is intended only as a guide to relevant legislation/policy. The information is accurate as at the date listed at the bottom of the page, but may be subject to change. To discuss individual circumstances please contact Services Australia.

2.4.1 The formulas


The Registrar will use an administrative formula to calculate the annual rate of child support payable under an administrative assessment, unless the assessment is varied by:

  • a departure order by a court (2.8.2)
  • a change of assessment decision (2.6), or
  • the provisions of a child support agreement (2.7.4).

Act reference

CSA Act section 5, Part 5 Division 2

What are the formulas?

The annual rate of child support payable is calculated using one of 6 formulas (section 35B). The 6 formulas are variations of one basic formula. To calculate a child support assessment, the formulas utilise:

  • the parents' incomes (2.4.4)
  • the percentage of care of the child, and the percentage of the costs met by this level of care (2.4.5)
  • the costs of the child (2.4.6).

For information about the tables and values used in the calculation of the assessments, refer to 2.4.2.

The formulas adopt an 'income shares' approach, where the costs of the children are calculated based on the parents' combined income and shared between the parents in proportion to each parent's share of their combined income. Either parent may meet some or all of their share of the costs during care.

A child support case is defined as the assessment for child support for all children who are children of both the parents of the child (section 5). If somebody other than a parent is caring for a child, then there may be 3 (or more) parties to a case.

In the majority of cases, the rate of child support will be assessed using Formula 1. The formulas are:

  • Formula 1 - the basic formula, for a single child support case (section 35). See 2.4.7.
  • Formula 2 - for a single child support case with a non-parent carer (section 36). See 2.4.8.
  • Formula 3 - if one or both of the parents have multiple child support cases (section 37). See 2.4.9.
  • Formula 4 - if a parent has multiple child support cases and there is a non-parent carer for one or more children (section 38). See 2.4.9.
  • Formula 5 - for a child support case with a non-parent carer, where one parent is not assessed because they are not a resident of Australia or there are special circumstances (section 39). See 2.4.10.
  • Formula 6 - for a child support case with a non-parent carer, where one parent is deceased (section 40). See 2.4.10.

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