Australian Child Support Calculator

Formula 1, step by step

Services Australia's Formula 1 (Child Support Assessment Act 1989, s35) runs each parent's income through 8 steps. Step 1: each parent's child support income is their Adjusted Taxable Income (ATI) minus the self-support amount (A$31,046 for CSP 1 January 2026, floored at $0 — you can't have negative child support income). Step 2 adds both parents' child support income together (combined child support income). Step 3 turns each parent's share of that combined figure into an income percentage.

Steps 4-5 convert care (nights per year with the child) into a cost percentage using the Care and Cost table below. Step 6 subtracts cost % from income % to get each parent's child support percentage — whoever's number is positive is the payer. Step 7 looks up the Costs of the Children (COTC) — what it actually costs to raise the children at that combined income — in one of 3 tables depending on the children's ages. Step 8 multiplies the payer's child support % by the COTC to get the annual amount. Neither the official estimator (processing.csa.gov.au) nor the private calculators we reviewed show this working — the calculator below does, at every step.

The Costs of the Children table, made readable

The COTC table answers: at this combined income, with this many children of these ages, what does it cost to raise them per year? There are 3 separate tables — all children 0-12, all children 13+, and mixed ages — because older children cost more. Rates climb through 5 income bands and then cap at a combined income of A$232,843 (2.5× annualised MTAWE, A$93,137) — above that, the COTC figure stops rising, even if income keeps climbing.

Example: at A$57,908 combined income, 1 child aged 0-12, the COTC is A$9,618/year. At a much higher, capped combined income of A$282,908 with 3 children of mixed ages, the COTC is capped at A$61,005/year (A$20,335 per child) — it would be higher without the cap. For 4 or more children in one case, the table still only goes up to "3 or more children": the total COTC from that row is divided by the actual number of children to get a cost per child.

Care % vs cost % — why 50/50 custody usually isn't $0

This is the single most common misunderstanding about Australian child support. A parent's care percentage (nights per year) doesn't translate 1-for-1 into a cost percentage — it goes through the Care and Cost table, which has flat bands (0%, 24%, 50%, 76%, 100%) and two sliding bands (35-47% and 53-65% care, where cost % increases 2 percentage points for every 1 point of care above the band's floor).

Worked example: Parent A earns A$70,000, Parent B earns A$65,000, 2 children, care split almost exactly 50/50 (183/182 nights). Both parents land in the 48-52% care band, so both get a flat 50% cost percentage — the care split cancels out. But their income percentages are 53.43% and 46.57% (not 50/50, because Parent A earns more). Since income % dominates when cost % is equalised, Parent A still owes A$591/year — small, but not $0. 50/50 overnight care equalises cost %, it doesn't erase the income gap. Genuine $0 only happens when income % and cost % are also equal (or very close).

Self-support amount and the income cap

Before anything else, A$31,046 of each parent's income (a third of annualised MTAWE, A$93,137) is set aside as their own cost of living — only income above that counts toward child support. A parent earning at or below the self-support amount contributes $0 child support income, not a negative figure.

At the other end, combined child support income above A$232,843 doesn't increase the COTC further — see the capped example above (A$57,996/year on A$282,908 combined income, capped from a higher uncapped figure).

What this v1 calculator doesn't cover

This tool implements Formula 1 only — a single child support case, care by the 2 legal parents alone, no non-parent carer (e.g. a grandparent), no other child support cases for either parent, and no relevant dependent children living with a parent. Services Australia calls these Formulas 2-6, and a Change of Assessment (a discretionary departure from the formula for special circumstances) isn't a formula at all — it's a case-by-case decision, so it's out of scope by nature.

Two flat-rate edge cases aren't auto-applied: the Fixed Annual Rate (A$1,825 per child per year, for some low-income or pension-recipient payers) and the Minimum Annual Rate (A$551/year). If your Formula 1 result comes out unusually low, the real assessment may substitute one of these instead — the calculator flags this in a warning but doesn't compute it.

Care is also assumed uniform across all children in the case (a simplification) — the real formula allows different nights arrangements per child if a family's circumstances genuinely differ child to child.