Plain-language summary for parents. Child support can feel impossible to predict because the final assessment depends on both parents’ incomes and the number of nights the children spend with each parent. But one part of the system is fixed and mathematical: the official “Costs of Children” schedule. That schedule is the cost base that the child support formula allocates between parents.
This article answers one question only: how much does a second or third child change the official cost base that drives child support? Using only the official 2026 Costs of Children tables, we show that a second child increases the assessed cost base by about 15% to 74%, depending on combined income band, the children’s ages, and which one-child baseline you are comparing against. Compared with one child, three or more children increases the cost base by about 29% to 115%.
Abstract
Parents often ask how having a second or third child changes child support. Although final assessments depend on income shares and care time, the official Costs of Children schedule is a fixed mathematical input to every administrative assessment. This article uses only the 2026 Costs of Children tables to quantify how the cost base changes with family size by combined parental income band and child age structure. We evaluate each income band at its upper bound and compute ratios of two-child and three-or-more-child costs relative to a one-child baseline.
Across income bands and family structures, a second child increases the assessed cost base by approximately 15% to 74%. Compared with one child, three or more children increases the assessed cost base by approximately 29% to 115%. The results demonstrate that costs rise substantially with additional children but do not scale proportionally, reflecting economies of scale embedded in the statutory schedule. The findings provide a transparent reference point for parents and practitioners grounded entirely in the official Australian framework.
1. Introduction
A child support assessment is not simply “a percentage of income.” It is a multi-step allocation of an official cost base between parents. That cost base is defined by the Costs of Children schedule, which varies by (a) the parents’ combined child support income, (b) the number of children, and (c) the age structure of the children. Because this schedule is an explicit mathematical object, it can be analysed directly.
This article isolates the effect of family size on the Costs of Children schedule. It does not attempt to compute an individual assessment (which would require care percentages and income shares). Instead, it answers “how does a second or third child change the cost base the formula is allocating?”
2. Data
The only data used in this article are the official 2026 Costs of Children tables, reported as piecewise functions by income band. For a timeless presentation, we label the income ranges as Income Band 1 through Income Band 5, while still reporting the official 2026 thresholds and marginal rates.
2.1 Income bands (2026 thresholds)
- Income Band 1: $0 to $46,569
- Income Band 2: $46,570 to $93,137
- Income Band 3: $93,138 to $139,706
- Income Band 4: $139,707 to $186,274
- Income Band 5: $186,275 to $232,843
3. Methods
3.1 Evaluating each band
The official tables are piecewise by income band. Within each band, the table implies a base amount plus a marginal rate (for example, “12 cents for each $1 over the band threshold”). To make bands comparable, we evaluate the cost at the upper bound of each band. This yields a single representative cost level per band and avoids ambiguity caused by different incomes inside the same band.
3.2 Ratio method
For each income band and age structure, we compute ratios:
- Second-child multiplier = Cost(2 children) / Cost(1 child)
- Second-child increase = Multiplier − 1
- Third-child multiplier (3+ schedule) = Cost(3+ children) / Cost(1 child)
- Third-child increase = Multiplier − 1
For mixed ages, the official schedule begins at two children. To integrate mixed ages into one-child comparisons, we report two baselines: (a) one young child as the reference, and (b) one teen as the reference.
4. Official 2026 Costs of Children schedules (marginal rates)
4.1 Children aged 12 and under (marginal rates per $1 of combined income)
| Income band | 1 child | 2 children | 3+ children |
|---|---|---|---|
| Income Band 1 ($0 to $46,569) | 17c | 24c | 27c |
| Income Band 2 ($46,570 to $93,137) | 15c | 23c | 26c |
| Income Band 3 ($93,138 to $139,706) | 12c | 20c | 25c |
| Income Band 4 ($139,707 to $186,274) | 10c | 18c | 24c |
| Income Band 5 ($186,275 to $232,843) | 7c | 10c | 18c |
Cap (2026): above $232,843, costs do not increase further. Maximum annual costs are $28,407 (1 child), $44,241 (2 children), and $55,882 (3+ children).
4.2 Children aged 13 or older (marginal rates per $1 of combined income)
| Income band | 1 child | 2 children | 3+ children |
|---|---|---|---|
| Income Band 1 ($0 to $46,569) | 23c | 29c | 32c |
| Income Band 2 ($46,570 to $93,137) | 22c | 28c | 31c |
| Income Band 3 ($93,138 to $139,706) | 12c | 25c | 30c |
| Income Band 4 ($139,707 to $186,274) | 10c | 20c | 29c |
| Income Band 5 ($186,275 to $232,843) | 9c | 13c | 20c |
Cap (2026): above $232,843, costs do not increase further. Maximum annual costs are $35,392 (1 child), $53,544 (2 children), and $66,128 (3+ children).
4.3 Children of mixed ages (2 children and 3+ children only, marginal rates per $1 of combined income)
| Income band | 2 children (mixed) | 3+ children (mixed) |
|---|---|---|
| Income Band 1 ($0 to $46,569) | 26.5c | 29.5c |
| Income Band 2 ($46,570 to $93,137) | 25.5c | 28.5c |
| Income Band 3 ($93,138 to $139,706) | 22.5c | 27.5c |
| Income Band 4 ($139,707 to $186,274) | 19c | 26.5c |
| Income Band 5 ($186,275 to $232,843) | 11.5c | 19c |
Cap (2026): above $232,843, costs do not increase further. Maximum annual costs are $48,897 (2 children mixed) and $61,005 (3+ children mixed).
5. Results
The tables below report relative cost levels by income band, evaluated at the top of each band. They answer the central question: how much larger is the official cost base when a family has a second child or a third child?
5.1 Children aged 12 and under (baseline: 1 young child = 1.00)
| Income band | 1 child | 2 children | 3+ children |
|---|---|---|---|
| Income Band 1 | 1.00 | 1.41 | 1.59 |
| Income Band 2 | 1.00 | 1.47 | 1.66 |
| Income Band 3 | 1.00 | 1.56 | 1.82 |
| Income Band 4 | 1.00 | 1.57 | 1.89 |
| Income Band 5 | 1.00 | 1.56 | 1.97 |
5.2 Children aged 13 or older (baseline: 1 teen = 1.00)
| Income band | 1 child | 2 children | 3+ children |
|---|---|---|---|
| Income Band 1 | 1.00 | 1.27 | 1.39 |
| Income Band 2 | 1.00 | 1.26 | 1.40 |
| Income Band 3 | 1.00 | 1.44 | 1.63 |
| Income Band 4 | 1.00 | 1.52 | 1.82 |
| Income Band 5 | 1.00 | 1.51 | 1.87 |
5.3 Mixed ages (baseline: 1 young child = 1.00)
| Income band | 1 young child | 2 children (mixed) | 3+ children (mixed) |
|---|---|---|---|
| Income Band 1 | 1.00 | 1.56 | 1.74 |
| Income Band 2 | 1.00 | 1.63 | 1.81 |
| Income Band 3 | 1.00 | 1.74 | 2.00 |
| Income Band 4 | 1.00 | 1.73 | 2.07 |
| Income Band 5 | 1.00 | 1.72 | 2.15 |
5.4 Mixed ages (baseline: 1 teen = 1.00)
| Income band | 1 teen | 2 children (mixed) | 3+ children (mixed) |
|---|---|---|---|
| Income Band 1 | 1.00 | 1.16 | 1.29 |
| Income Band 2 | 1.00 | 1.15 | 1.29 |
| Income Band 3 | 1.00 | 1.31 | 1.50 |
| Income Band 4 | 1.00 | 1.40 | 1.67 |
| Income Band 5 | 1.00 | 1.38 | 1.72 |
6. Definitive percentage ranges
Using the ratio method above (evaluated at the upper bound of each income band), the official 2026 schedule implies the following ranges.
- Second child (two children vs one child): from about 15% (mixed ages measured relative to a one-teen baseline) up to about 74% (mixed-age families relative to a one-young-child baseline).
- Third child (three or more children vs one child): from about 29% (mixed ages measured relative to a one-teen baseline) up to about 115% (mixed-age families relative to a one-young-child baseline).
7. Discussion
The Costs of Children schedule is intentionally non-linear. A second child increases the cost base substantially, but the total cost does not double. A third child increases the cost base further, but again with diminishing proportional increases. This pattern is consistent with economies of scale embedded in the schedule, where some household costs are shared across children.
Age composition matters. Teen schedules are higher in absolute terms, but the proportional increase from one to two children can be smaller in the lower income bands. Mixed-age families show the largest proportional increase when the baseline is one young child, because the two-child mixed schedule reflects the presence of a teenager.
8. Limitations
This article measures changes in the official Costs of Children schedule only. Individual child support assessments depend on additional variables, including each parent’s income share and care percentage. However, holding those additional variables constant, the percentage change in the cost schedule is the cleanest mathematical measure of how additional children change the cost base that drives assessments.
9. Conclusion
Using only the official 2026 Costs of Children tables, we provide a definitive mathematical answer to how a second or third child affects child support cost foundations. Depending on combined income band and family structure, a second child increases the assessed cost base by approximately 15% to 74%. Compared with one child, three or more children increases the assessed cost base by approximately 29% to 115%.
These findings explain why child support can rise sharply when family size increases, while still increasing by less than proportional multiples.
Appendix A. Worked example (illustrative only)
Within a given income band, the official table expresses costs as a base amount plus a marginal rate on income above the band threshold. For example, in the teenagers table for Income Band 3 ($93,138 to $139,706), the marginal rates are 12c per $1 (one child) and 25c per $1 (two children). Evaluating at the upper bound produces a single cost for that band, which is then used to form the ratios in Section 5.
