One of the most counterintuitive moments in family law is when a child turns 18 and leaves home — and the remaining child support obligation increases per child. This happens because the federal child support tables are not simply divided by the number of children. The table amount for two children is not double the amount for one child; the per-child rate changes at each level. When a family goes from two children to one, the total amount decreases, but the amount per remaining child can go up significantly.

How the Table Recalculates

The federal child support tables have separate columns for 1, 2, 3, 4, 5, and 6+ children. When a child is no longer a dependent, the table amount is looked up for the new number of children — it is not simply reduced by a fraction. For example, at $85,000 income in Ontario:

When the family goes from 3 to 2 children, the total drops by $435 but the per-child amount increases from $577 to $648. Going from 2 to 1, the total drops by $465 but the per-child amount jumps from $648 to $831 — a 28% increase per remaining child. This is not a bug; the tables are designed this way because a household with fewer children has lower fixed costs (housing, utilities) to spread across.

Support Does Not End Automatically

A child turning 18 (or 19 in some provinces) does not automatically terminate child support. The existing court order or agreement remains in force until it is formally varied by the court or the parties agree in writing to change it. A payor who simply stops paying creates arrears — which accrue interest and can be enforced through garnishment, licence suspension, or contempt proceedings.

To change the support amount, the payor must either: (a) negotiate a new agreement with the other parent, reflecting the reduced number of dependent children, or (b) apply to the court for a variation under section 17 of the Divorce Act, demonstrating a material change in circumstances (a child reaching the age of majority and becoming independent qualifies).

University Students and Ongoing Dependency

A child who turns 18 but remains financially dependent — typically because they are pursuing post-secondary education — may still qualify as a "child of the marriage" under the Divorce Act. In these cases, support may continue even though the child is legally an adult. Courts assess whether the child is pursuing a reasonable education, maintaining adequate academic progress, and remains unable to withdraw from the parents' charge. For a full discussion, see our article on support for adult children.

Impact on Spousal Support

When a child becomes independent, the spousal support calculation may also change. Under the SSAG with-child formula, child support is deducted from each parent's income before INDI is calculated. A lower child support amount means more income is available in the INDI pool, which can change the spousal support range. Additionally, the duration framework shifts: with fewer (or no) dependent children, the with-child formula may give way to the without-child formula, with different duration rules.

Running the calculator with the new number of children shows the updated combined picture — both the new child support amount and the revised spousal support range.