Every child support assessment in Australia is built on two key figures from the Child Support Guide. The Self-Support Amount (SSA) is the income floor each parent gets to keep before child support is calculated — set at one-third of Male Total Average Weekly Earnings, which is $31,046 for 2026. The Cost of Children (COTC) table is the Government's published estimate of how much it costs to raise children at different combined-income levels — it's the figure both parents are then asked to contribute towards. Understanding these two figures is the difference between accepting a child support assessment and questioning it intelligently.
This guide explains both — what they are, how they're calculated, and how they shape the dollar amount you pay or receive.
The Self-Support Amount (SSA) — what each parent keeps first
Before child support is calculated, each parent gets to keep an amount of income to support themselves. That floor is the Self-Support Amount.
For 2026: $31,046 (effective from 1 January 2026).
The SSA is calculated as one-third of the annualised Male Total Average Weekly Earnings (MTAWE) — a figure published by the Australian Bureau of Statistics. For 2026:
- MTAWE (annualised) = $93,137
- SSA = ⅓ × $93,137 = $31,046
This figure indexes annually with MTAWE:
| Year | MTAWE | SSA |
|---|---|---|
| 2024 | $85,389 | $28,463 |
| 2025 | $89,523 | $29,841 |
| 2026 | $93,137 | $31,046 |
Why it exists: the rules recognise that every parent — whether they're paying or receiving — needs enough income to support themselves before they can support a child. Without the SSA, low-income parents could be left with nothing.
How the SSA shapes your assessment
In the 8-step CSA formula, the SSA is the first thing subtracted from each parent's ATI to produce their Child Support Income (CSI):
Child Support Income = ATI − Self-Support Amount − Relevant Dependent Children Allowance
So a parent earning $50,000 has CSI of $50,000 − $31,046 = $18,954. A parent earning $31,046 or less has CSI of $0 — they're not assessed for any positive child support contribution (though minimum-rate rules may still apply).
The SSA is the same for both parents — paying parent and receiving parent — regardless of who pays whom.
Try the free CS calculator → · Open the full estimator → Enter both incomes and the estimator applies the current SSA, COTC bands, and the full 8-step formula.
The Cost of Children (COTC) table
The COTC table is the Government's published estimate of how much it costs Australian parents to raise children at different combined-income levels. It's published in Child Support Guide §2.3.1.20 and indexed annually.
The table has 8 rows (combined CSI bands from $0 to ≥3.5 × MTAWE) and 6 columns (1 child age 0-12, 2 children 0-12, 3+ children 0-12, 1 teen, 2 teens, 3+ teens). The cells are piecewise linear functions — for each band, the COTC is calculated as:
COTC = baseline cost + (marginal cents ÷ 100) × (CSI − bandStart)
The last band is a cap (marginalCents = 0) — above that combined income, the Government doesn't assume further per-dollar spending on children for CS purposes.
Worked example — combined CSI $62,908, one child aged 6
Following the example from the 8-step formula guide, where two parents had:
- Parent A ATI $80,000 → CSI $48,954
- Parent B ATI $45,000 → CSI $13,954
- Combined CSI = $62,908
At combined CSI $62,908 with one child aged 6, the COTC formula gives approximately $11,300/year (the exact figure depends on the specific band; NestWise's estimator pulls the precise cell from the 2026 table).
Each parent is then expected to contribute proportionally:
- A's income share (77.82%) → A's expected contribution = 77.82% × $11,300 ≈ $8,792/year
- B's income share (22.18%) → B's expected contribution = 22.18% × $11,300 ≈ $2,508/year
The next step (cost percentage based on care) determines who actually pays whom — but the COTC sets the dollar pool both parents are working from.
Two key rules about COTC
1. COTC scales with combined income, not yours alone. Two families with the same children can have very different COTCs if their combined incomes differ. The Government's logic: higher-income families spend more on children, so the per-child cost the assessment uses is higher.
2. COTC scales with the number AND ages of children. Teenagers (13+) are assumed to cost more than younger children (0-12). Adding a second or third child increases the COTC, but not linearly — the per-child cost decreases as families spread fixed costs (housing, utilities) across more kids.
The two figures together
The SSA + COTC pair is what makes the Australian system distinctive:
- SSA says: "Both parents keep a baseline."
- COTC says: "The cost of raising children is a known amount we'll work from."
The 8-step formula then asks each parent to contribute toward the COTC in proportion to their income share (after SSA), with the care percentage adjusting who actually transfers money to whom. Nothing in the formula asks one parent to fund the other's lifestyle — it asks both parents to contribute proportionally to the cost of the kids.
The four gotchas that catch families out
1. The SSA indexes every January, not every July. Most family-payment rates change at the start of the financial year (1 July). Child support's SSA changes at the start of the calendar year (1 January) — different indexation cycle.
2. COTC depends on combined ATI, not just yours. If your ex-partner has a significant income change, your COTC changes too — even though your own income hasn't moved. NestWise updates the estimator's COTC table within weeks of each annual publication.
3. The "relevant dependent children" allowance is a separate deduction. If you have children in your current household (whether biological, adopted, or step), there's an additional allowance that further reduces your CSI before the formula runs. This is on top of the SSA. The NestWise estimator handles this automatically.
4. The COTC cap matters at high incomes. Combined CSI above the last band (≈3.5 × MTAWE for 2026 = $325,980) doesn't increase the COTC further. High-income families effectively top out — additional income above the cap doesn't increase the assessment.
How NestWise applies these figures
NestWise's Child Support Estimator uses the exact SSA and COTC table for the current calendar year:
- SSA — pulled from
lib/rates.tsCHILD_SUPPORT_VALUES_BY_YEAR(2026: $31,046). - COTC table — the full 8-row × 6-column table from CSG §2.3.1.20.
- Income share + care share — applied to the COTC to compute pay/receive amounts.
- MAR / FAR floors — applied if the formula result is below the minimums.
The whole engine is locked behind 198 regression tests. The full source list is on the sources page.
What to read next
- How is child support worked out? The 8-step formula in plain English — the full formula in context.
- Maintenance Action Test — why your CS arrangement affects FTB-A — the income-test interaction between CS received and FTB Part A.
- What is Adjusted Taxable Income (ATI)? — the income figure that feeds the CSA formula.