Child Support Calculator

The Child Support Calculator estimates support in two completely different ways, and it is important to pick the right one. Simple mode applies a flat percentage of one parent's monthly income based on how many children there are. CSA mode runs the full Australian Child Support Agency formula — both parents' incomes, a self-support reserve, care nights, and the official Costs of Children table — and produces a complete breakdown with the payer, the receiver, and weekly, fortnightly, monthly, and annual amounts.

This is an educational estimator. Courts and agencies determine actual obligations; it is not legal advice.

Mode 1: Simple Percentage Estimate

$\text{Monthly Support} = \text{Monthly Income} \times \text{Percentage}$

| Children | Percentage of monthly income | |---|---| | 1 | 20% | | 2 | 25% | | 3 | 30% | | 4 | 35% | | 5 or more | 40% |

Example: a monthly income of **$6,000** with **2 children** gives 25$6,000 = **$1,500 per month**, or$18,000 per year. Use this mode for a fast first approximation or where a flat-percentage guideline is genuinely the local rule — and understand that it ignores the other parent's income and care arrangements entirely.

Mode 2: The Australian CSA Formula, Step by Step

The CSA formula equalizes children's living standards across both households. Its steps, exactly as the calculator performs them:

1. Self-support amount: each parent keeps the first **$29,841** of annual income (the 2025 Services Australia reserve). Child support income = income −$29,841, floored at zero.
2. Income shares: each parent's child support income ÷ the combined total.
3. Care percentage: care nights ÷ 365 (Parent B gets the remaining nights).
4. Cost percentage: care is converted to a cost share via the official bands — under 14% care carries 0% of costs, 14 to 34% carries 24%, 35 to 47% scales from 25% upward, 48 to 52% is 50%, 53 to 65% scales from 51%, 66 to 86% is 76%, and above 86% is 100%.
5. Costs of Children: the combined child support income is run through the Costs of Children table — marginal rates per income bracket, by age group (0–12, 13+, mixed) and number of children (capped at 3) — with combined income capped at $223,808.
6. Child support percentage: income share − cost share. The parent with the positive percentage pays that percentage of the Costs of Children to the other.

Worked Example (CSA Mode)

Parent A earns $90,000**, Parent B earns **$50,000; 2 children aged 0–12; Parent A has 100 care nights a year (Parent B, 265):

• Child support incomes: A = $90,000 −$29,841 = $60,159**; B = **$20,159; combined $80,318.
• Income shares: A 74.9%, B 25.1%.
• Care: A 27.4% → cost percentage 24%; B 72.6% → cost percentage 76%.
• Child support percentages: A = 74.9 − 24 = +50.9%; B = 25.1 − 76 = −50.9%. Parent A pays.
• Costs of Children on $80,318 (2 children, 0–12): first$44,762 at 24% = $10,742.88, remaining$35,556 at 23% = $8,177.88 — total **$18,921**.
• Annual support = 50.9% × $18,921 =$9,631 — monthly $803**, fortnightly **$370, weekly $185, paid by Parent A to Parent B.

Notice how different this is from the simple mode: the paying parent's own care nights and the other parent's income both reduce the transfer, because the formula assumes each parent is already funding the children directly during their care time.

Limitations

CSA mode implements the standard administrative formula with the 2025 self-support amount and the standard Costs of Children table; it does not cover departures, change-of-assessment reasons, non-parent carers, or multi-case allowances, and the age-group table caps at three children. Simple mode is a generic flat-percentage heuristic used by some jurisdictions in simplified form — it is not the US state guideline calculation, which uses income-shares tables this tool does not reproduce. Verify any figure against your official agency assessment or a family lawyer before relying on it.

Frequently Asked Questions

Does 50/50 care mean nobody pays?

Not under the CSA formula. At equal care both parents have a 50% cost percentage, but if one earns more, their income share exceeds 50% and they still pay the difference — equal time does not mean equal cost-bearing when incomes differ.

Why does the first $29,841 of income not count?

That is the self-support amount: the formula reserves enough income for each parent to support themselves before any income is counted toward the children. Only income above the reserve enters the combined pool, which is why a low-earning parent's share drops quickly.

Which mode should I use?

If you are in Australia and want the realistic administrative estimate, use CSA mode with both incomes and actual care nights. Use simple mode only for a rough single-income percentage estimate, and never mix the two — they model different legal systems.