How is child support calculated in Australia?

Services Australia runs an 8-step formula. Each parent's Child Support Income equals their adjusted taxable income minus the Self-Support Amount ($31,046 for 2026). Both parents' CSIs combine, each gets an income percentage and a care percentage, the care % maps to a cost percentage, and the formula calculates the transfer based on the higher-earning / lower-caring parent's "share gap". The result is then checked against the Minimum Annual Rate floor + Fixed Annual Rate floor.

What's the minimum child support payment?

Two floors apply. The Minimum Annual Rate (MAR) for 2026 is $519 per case — the absolute minimum payable when the formula says less. The Fixed Annual Rate (FAR) is $1,720 per child (capped at 3 children, so $5,160 max) — applies when the paying parent has zero income support, ATI below the Pension PP Single ($25,225), and less than 35% care. FAR is checked FIRST and overrides the formula result when it applies.

Does my new partner's income affect my child support?

No. The child support formula uses only the two biological/adoptive parents' incomes. New partners, de facto spouses, step-parents — none of them enter the formula. (Centrelink family payments work differently — your new partner's income DOES count for FTB and CCS purposes.) See our step-parent income guide for the full detail.

What is the Self-Support Amount?

It's the floor each parent gets to keep before any child support is calculated. For 2026 the SSA is $31,046. Your ATI minus SSA equals your Child Support Income — the figure the formula actually uses. Indexed each year to inflation. Same SSA applies to both paying and receiving parents.

What's the Cost of Children table?

A government-published lookup table that estimates the annual cost of raising children at each income band. The 8-step formula uses it to calculate the total cost of the children at the combined parental income, then divides that cost between parents based on their respective income and care percentages. Higher combined income means higher assumed cost per child.

Can I change my child support assessment?

Yes, via the Change of Assessment process — 10 specified grounds (high contact costs, special needs, education costs, child has own income, parent has financial resources beyond declared income, earning capacity differs from declared, etc.). It's free, decisions usually within 90 days, and you can appeal to the Administrative Appeals Tribunal if dissatisfied. See our Change of Assessment guide for the full process.

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How much child support will you pay or receive?

2026-27 rates. Four questions, 60 seconds. Services Australia Formula 1 with the Cost of Children table — same engine the dashboard estimator uses.

This is the basic CS calc (Formula 1) — same depth as Services Australia's own. NestWise's full estimator adds:

Per-child care % — when kids spend different amounts of time with each parent
Shared-care % impacts across CS, FTB-A, and CCS in one view
FTB-A Maintenance Income Test integration — rare in competitor calcs; shows the real impact on your fortnightly FTB-A
RDCA — kids from a new relationship reduce your CS income
Multi-case Formula 3 — when a parent has more than one CS case
Assessment scanner (PDF/photo OCR) + Expense tracker with NAP classification
Extra Day calculator — and how more income changes your CS + FTB-A together

Your annual gross income

Adjusted taxable income for child support (taxable income + reportable fringe benefits + reportable super + tax-free pensions + investment losses + foreign income). For PAYG workers, your salary is usually close.

Other parent's annual gross income

Their ATI for child support. If you don't know exactly, use a best guess — Services Australia uses both parents' ATO-assessed figures in the actual assessment.

How many children in this case?

How old are they?

Cost of Children jumps at age 13. Pick the band that matches most of your kids.

Your share of care: 50% (theirs: 50%)

0% (no care)50% (shared)100% (sole care)

Percentage of nights the kids spend with you over a typical fortnight. The formula treats this as the same % for every child in the case (the playground simplification).

Last step — 30 seconds.

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Not financial advice · Services Australia 2026-27 figures · NestWise estimate, verify before relying on it · Services Australia

How this calculator is built

Built by an Australian Chartered Accountant— the same engine the paid NestWise dashboard runs. Every rate table is verified against the source below every 1 July, 20 March, and 20 September and covered by a 200+ scenario regression suite.

Sources: Services Australia — Child Support Assessment

What goes into your child support estimate

Australian child support uses Formula 1 from the Child Support (Assessment) Act 1989. Five inputs drive the figure:

Both parents’ taxable income — from the most recent tax assessment. Not gross salary; taxable income (after pre-tax deductions like salary sacrificed super).
Number of children — and their ages, because the cost-of-children percentage shifts at ages 13 and 18.
Care arrangement — the percentage of nights each parent has the child. Counted as overnights ÷ 365.
Other dependent children — biological children either parent is supporting elsewhere reduce the assessment.
The self-support amount — a fixed annual figure (currently ~$31,046, indexed) that’s subtracted from each parent’s income before the formula runs.

Worked example: how the maths actually runs

Mark and Sara have one child, age 8. Mark earns $90,000 taxable; Sara earns $50,000 taxable. Sara has 70% care (256 nights), Mark has 30% care (109 nights).

Step 1: Adjusted income for each parent

Subtract the $31,046 self-support amount from each:
Mark: $90,000 − $31,046 = $58,954
Sara: $50,000 − $31,046 = $18,954

Step 2: Combined Child Support Income

$58,954 + $18,954 = $77,908. Each parent’s percentage of the combined income: Mark 75.7%, Sara 24.3%.

Step 3: Look up the Cost of Children

On combined income of $77,908for one child aged 0–12, the Cost of Children table says ~$13,500/year (illustrative). Mark’s 75.7% share = $10,216/yr. Sara’s 24.3% share = $3,284/yr.

Step 4: Apply care percentage

Care percentage is converted to a cost percentage. Sara’s 70% care = 76% cost. Mark’s 30% care = 24% cost. Each parent’s assessed contribution is their income-share cost MINUS what they already pay through care:
Mark owes $10,216 − (24% × $13,500 = $3,240) = $6,976/year.
Sara owes $3,284 − (76% × $13,500 = $10,260) = $0 (floored at zero).

Step 5: Direction + assessment

Mark pays Sara $6,976/year (~$268/fortnight). Sara collects.

Run your own numbers in the calculator above. The same Formula 1 applies, with the Services Australia rate tables for 2026-27.

How NestWise compares to the Services Australia calculator

The official Services Australia online estimator and the NestWise calculator both implement Formula 1 from the same legislation. The figures should match within rounding.

What NestWise surfaces that the official tool doesn’t

Direction inline — we tell you who pays whom without you re-running the calc. The official tool only returns a figure.
Per-fortnight equivalent — alongside the annual figure, so you can budget against your pay cycle.
Shared-care impact visible — tweak the care % slider and watch the figure move live. The official tool requires re-submission.
Full Cost of Children table referenced — you see the income band + age band the formula landed on.

What NestWise doesn’t model

Change of Assessment decisions — if Services Australia has overridden the formula due to special circumstances, your real assessment can differ.
Care percentage overrides set by the Registrar (rare but possible).
Lump-sum debts or arrears collection — the calc is for the ongoing assessment only.

For everyday scenario-running (“what if I drop a day?”, “what if care changes to 50/50?”), NestWise is faster. For a binding agreement or a formal application, verify against your actual Services Australia assessment.

Five common mistakes when estimating child support

Using gross salary instead of taxable income. The formula reads taxable income from your tax assessment notice. Salary-sacrificed super, work-related deductions, and rental losses all reduce the figure the formula sees.
Forgetting the self-support amount. Each parent gets ~$31,046 subtracted before the formula runs. If you’re earning that or less, your assessed contribution is effectively zero.
Misreading care percentage. Care is overnights ÷ 365. 50/50 care = 50%. Every-second-weekend = ~14%. Half the school holidays + alternating weekends = ~30%.
Not counting other biological children. If either parent has a dependent biological child elsewhere (not the step-children of a new partner), that reduces their assessable income and shifts the figure.
Assuming high income means a huge debt. The Cost of Children table caps the cost at a Maximum Income (currently ~$232,843 combined). Income above that doesn’t lift the assessment.

When to re-estimate your child support

Centrelink uses your last lodged tax assessment by default, updated each year after EOFY. You can ask for a re-estimate during the year when:

Income changed by 15% or more — pay rise, job loss, dropping to part-time, taking parental leave.
Care arrangement changed — a custody adjustment, kids moving in or out, school-holiday redistribution.
A new dependent child arrived — biological child with a new partner counts; step-children don’t.
Self-employed income looks different to last year — the assessment runs on a year-old taxable figure that may bear no relation to this year’s reality.

Run the calculator above with your new numbers first, then read the full formula guide before lodging a formal change-of-assessment request.

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Per-child care % when kids spend different time with each parent
Kids from a new relationship (RDCA) reduce your CS income
Multi-case Formula 3 when a parent has more than one CS case
FTB-A Maintenance Income Test integrated — see CS + FTB-A together
Assessment scanner + expense tracker saved to your profile

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