Child support isn't arbitrary — it's a formula
If you're separating and have children, one of the first financial questions you'll face is child support. Many parents assume the amount is negotiated or decided by a judge, but in the vast majority of cases, it's calculated by Services Australia using a fixed 8-step formula set out in the Child Support (Assessment) Act 1989.
Understanding this formula removes a lot of the anxiety. It's not someone's opinion — it's maths. And once you know the inputs, you can estimate the outcome before you even apply. Use our Child Support Estimator to run your own numbers.
Important: This article is general information only. It is not legal or financial advice. For advice specific to your situation, contact Services Australia or a family lawyer.
The 8-step formula explained
Step 1: Work out each parent's child support income
Services Australia starts with each parent's adjusted taxable income. This includes taxable income, reportable fringe benefits, target foreign income, net investment losses, and certain tax-free pensions or benefits. For most PAYG employees, it's close to your gross salary minus tax deductions.
If you're self-employed, the calculation can be more complex — Services Australia may look at your tax return over multiple years or make an income estimate based on earning capacity.
Step 2: Subtract the self-support amount
Each parent gets a deduction called the self-support amount — a figure that represents the minimum you need to support yourself. For the 2025-26 financial year, this amount is $28,463 (indexed annually, based on one-third of the annualised Male Total Average Weekly Earnings).
If Parent A earns $95,000 and Parent B earns $55,000:
- Parent A's child support income: $95,000 - $28,463 = $66,537
- Parent B's child support income: $55,000 - $28,463 = $26,537
Step 3: Work out the combined child support income
Add both parents' child support incomes together:
$66,537 + $26,537 = $93,074 combined child support income.
Step 4: Work out each parent's income percentage
Each parent's share of the combined income determines how much of the children's costs they're responsible for:
- Parent A: $66,537 / $93,074 = 71.5%
- Parent B: $26,537 / $93,074 = 28.5%
Step 5: Work out each parent's care percentage
This is where the number of nights matters. Services Australia converts nights of care into a percentage, which is then placed into one of five care bands. Use our Care Percentage Calculator to see exactly where you fall.
The five care bands are:
| Nights per year | Care % | Cost % |
|---|---|---|
| 0-51 | 0-13% | Nil |
| 52-127 | 14-34% | 24% |
| 128-175 | 35-47% | 25% + 2% for each % above 35% |
| 176-255 | 48-52% | 25% + 2% for each % above 35% |
| 256-365 | 53-100% | 25% + 2% for each % above 35% |
Step 6: Work out each parent's child support percentage
Subtract each parent's cost percentage (from the care table) from their income percentage (from Step 4). A positive result means that parent pays child support. A negative result means they receive it.
Step 7: Work out the costs of the children
Services Australia uses a Costs of Children Table that varies by the combined child support income, the number of children, and their ages (under 13, 13 and over, or mixed). For example, for two children under 13 with a combined income of $93,074, the costs of the children might be around $24,500 per year.
These tables are updated annually and are available on the Services Australia website.
Step 8: Calculate the child support amount
Multiply the paying parent's child support percentage (Step 6) by the costs of the children (Step 7). That gives you the annual child support liability, which is then divided into monthly, fortnightly, or weekly payments.
The minimum annual rate
Even if the formula produces a very low amount, there's a minimum annual rate of child support. For 2025-26, this is $540 per year (roughly $10.38 per week). This applies when the paying parent has a very low income or is on Centrelink payments.
What income cap applies?
There's also a cap on the income used in the formula. For 2025-26, the child support income cap is approximately $191,000 (2.5 times the annualised Male Total Average Weekly Earnings). Income above this cap is not included in the formula — though the receiving parent can apply for a change of assessment if they believe the paying parent's lifestyle suggests a higher contribution is appropriate.
Private agreements vs assessments
You don't have to use the formula. Parents can make their own arrangements:
- Child support agreement: A formal agreement registered with Services Australia. Can be a binding agreement (requires independent legal advice for both parties) or a limited agreement (must be at least the assessed amount).
- Private arrangement: An informal agreement between parents. Not enforceable through Services Australia if one party stops paying.
If you go with a private arrangement, it's worth running the formula anyway so you know what the baseline would be. The Child Support Estimator gives you that number in seconds.
When assessments change
Your child support assessment isn't set in stone. It can change when:
- Either parent's income changes significantly
- Care arrangements change (even a shift of one night per week can move you between care bands)
- A child turns 13 (the costs of children table changes)
- A child leaves the assessment (turns 18 or finishes secondary school)
- Either parent has another child from a new relationship
If your circumstances have changed, read our guide on how to change your child support assessment and use the Child Support Change Calculator to estimate the impact.
Where to get help
Child support can be one of the most stressful parts of separation. These free resources can help:
- Services Australia: servicesaustralia.gov.au/child-support or call 131 272
- Family Relationship Advice Line: 1800 050 321 (free call, available 8am-8pm Mon-Fri, 10am-4pm Sat)
- Legal Aid: Available in every state and territory for eligible parents
For a full list of free services, see our guide to free separation resources in Australia.
