Child Support Formula Australia [2026]

Australia uses a standardised formula to calculate child support. It's the same calculation applied to every family across the country — your income, the other parent's income, how many children you have, and how much time each parent spends with them. Understanding the formula means you can check whether your assessment is correct and anticipate what will change if your circumstances do.

Why Does Australia Use a Formula?

Before the formula was introduced under the Child Support (Assessment) Act 1989, child support amounts were decided case by case — often by courts, and often inconsistently. The same family circumstances could produce very different outcomes depending on the judge or the state. The formula was introduced to make the system transparent, predictable, and consistent.

The formula doesn't account for everything. Private school fees, extracurricular activities and other non-standard costs require a separate agreement. But it provides a solid, enforceable baseline.

The 8-Step Formula — Broken Down

Step 1: Adjusted Taxable Income

Each parent's income is calculated as Adjusted Taxable Income (ATI) — not just the figure on your tax return. ATI includes salary, rental income, reportable fringe benefits, reportable super contributions, net investment losses (added back), and foreign income. A new partner's income is never included.

Step 2: Subtract the Self-Support Amount

Each parent keeps $31,046 (2026 figure) as a self-support amount — the income needed to support themselves. This is subtracted from each parent's ATI to get their Child Support Income (CSI). If a parent's ATI is below $31,046, their CSI is zero and they have minimal or no payment obligation.

Step 3: Combined Child Support Income

Both parents' CSI figures are added together to give the Combined Child Support Income (CCSI). This is the pool of income both parents contribute from.

Step 4: Income Percentage

Each parent's share of the CCSI is calculated as a percentage. This determines how much of the children's costs each parent is proportionally responsible for.

Step 5: Costs of the Children

Services Australia uses a published expenditure table that estimates annual child-rearing costs based on the combined income and number of children. The table is updated annually.

Note: These are illustrative figures only. Use the calculator for your specific assessment.

Step 6: Each Parent's Cost Contribution

Each parent's income percentage is applied to the total cost figure to determine their expected contribution. A parent with a 70% income share contributes 70% of the costs.

Step 7: The Care Cost Offset

A parent who spends time with the child is already covering costs directly — food, housing, transport, activities during that time. Services Australia calculates a care percentage and offsets this against each parent's contribution. More care time means a larger offset.

Step 8: The Transfer Amount

The final figure is the difference between what each parent is expected to contribute and what they are already spending through direct care. The parent whose expected contribution exceeds their direct care spending pays the difference to the other parent.

What the Formula Doesn't Account For

The formula is a baseline, not a comprehensive accounting of every child-related cost. It doesn't include:

• Private school fees
• Extracurricular activities (sport, music, tutoring)
• High medical or dental costs above standard
• Overseas travel with the child

These require a private agreement or a Change of Assessment. See our guides on what child support doesn't cover and private agreements.

When the Formula Doesn't Apply

The formula applies by default to all Services Australia assessments. However, parents can replace it with a private agreement (Limited or Binding), and either parent can apply for a Change of Assessment if the formula produces an unfair result in their circumstances.

Common reasons for departing from the formula include a parent hiding income, a parent with high assets but low declared income, high special needs costs for a child, or significant travel costs for care time.