Shared Custody and the 40% Rule in Child Support

Shared parenting arrangements are common across Canada, and many separated parents assume that splitting time roughly evenly means child support disappears or is simply cut in half. That is not how the law works. A specific rule, often called the "40% rule," decides when a shared parenting arrangement changes how support is calculated, and even then the calculation is more nuanced than a straight split.

This page explains section 9 of the Federal Child Support Guidelines, how courts count parenting time, and why the Supreme Court of Canada's decision in Contino v Leonelli-Contino rejected any automatic formula. For background on the guideline tables themselves, see how the federal tables work. For costs that sit outside the basic table amount, such as childcare or medical costs, see special and extraordinary expenses.

What Section 9 Actually Says

Section 9 of the Federal Child Support Guidelines applies when each parent exercises "not less than 40% of parenting time" with a child over the course of a year. Once that threshold is met, the standard guideline table amount does not automatically apply. Instead, the amount of child support must be determined by considering three things together: the table amounts each parent would otherwise pay based on their income, the increased costs of a shared parenting arrangement, and the condition, means, needs, and other circumstances of each parent and child.

This is a real departure from how child support usually works. In a typical arrangement where one parent has the majority of parenting time, generally more than 60%, the paying parent's income is run through the applicable child support table and that is the end of the calculation, subject to special expenses. Section 9 replaces that mechanical approach with a broader, discretionary inquiry once shared parenting is established.

The 40% Threshold and How Time Is Counted

Forty percent of a year works out to roughly 146 days. Courts do not require an exact, mathematically equal split, but they do require proof of the actual amount of time a child is in each parent's care, not just what a written parenting plan says.

Time a child spends at school or in daycare while a parent has care and control still counts toward that parent's percentage. Some decisions look primarily at overnights as a practical proxy, others look at waking hours or a broader accounting of who is actually responsible for the child during a given block of time. There is no single formula that every court applies, and the parent relying on section 9 generally carries the burden of proving the threshold is met.

Because the difference between, say, 39% and 41% can change the entire calculation, this threshold is heavily litigated. A parenting schedule that looks close to even on paper can still fall short of 40% once summer vacations, extended-family time, or informal variations from the written plan are factored in. Parents relying on section 9 should keep a clear, contemporaneous record of the schedule actually followed, rather than assuming the parenting plan on paper will control.

How Support Is Calculated: Three Factors, Not a Formula

Once the 40% threshold is met, section 9 directs a court to weigh three factors together, rather than apply a single formula.

Table amounts for both parents. The usual starting point is to look up the table amount each parent would pay based on their own income, as if the other parent had the majority of parenting time, and offset the two figures against each other. This is often called the "set-off" amount. It gives the higher-income parent credit for the fact that they are directly covering costs during their own parenting time.

Increased costs of shared parenting. Maintaining two homes that are both set up for the child, with separate bedrooms, clothing, supplies, and activities, generally costs more in total than a single primary household paired with a visiting schedule. Courts look at whether the shared arrangement has actually increased overall costs and how those costs land on each parent.

Conditions, means, needs, and circumstances. This is the broadest factor. It allows a court to look at each parent's actual budget, any disparity in income, the child's needs, and whether a strict set-off would leave the child with a noticeably lower standard of living in one household than the other.

None of these three factors works as a rigid formula on its own. How the federal tables work explains the underlying provincial and territorial tables that feed into the first factor, but section 9 treats those tables as an input, not a final answer.

Contino v Leonelli-Contino: No Presumption of a Simple Set-Off

The leading authority on how these three factors interact is the Supreme Court of Canada's 2005 decision in Contino v Leonelli-Contino, 2005 SCC 63. The Court held that there is no presumption in favour of a simple set-off between the two table amounts, and no presumption that the guideline table amount continues to apply in full despite shared parenting.

The set-off calculation is only a starting point for the analysis. A court must go on to examine the actual increased costs of the shared arrangement and the real financial circumstances of both households, including any disparity in income between the parents. The Court described the underlying goal as a fair standard of support for the child and fair contributions from both parents, not a mechanical subtraction of two numbers.

In practice, this means a straight set-off can be adjusted upward or downward depending on the facts. A parent with significantly higher income may still be ordered to pay more than the bare set-off amount if a strict set-off would leave the child living at a noticeably lower standard in the lower-income household. There is no shortcut that avoids a genuine look at both parents' actual budgets.

Shared Custody vs Split Custody: Do Not Confuse the Two

Shared custody under section 9 is often confused with split custody under section 8 of the Guidelines, but the two address different situations.

Shared custody (section 9) applies when there is one child, or a group of children treated together, and each parent has that same child or children for at least 40% of the time. Split custody (section 8) applies when there are two or more children and each parent has the majority of parenting time with at least one of them, for example when an older child lives mainly with one parent and a younger sibling lives mainly with the other.

Split custody under section 8 uses a straightforward set-off: each parent's table amount is calculated as though a support order were sought against them for the child or children in the other parent's care, and the difference is paid by the parent with the higher figure. Unlike section 9, section 8 does not build in the same discretionary weighing of increased costs and individual circumstances. The two sections were designed for different fact patterns and should not be applied interchangeably.

Common Pitfalls in Shared Custody Support Disputes

A few recurring issues come up in shared custody child support disputes.

Falling just short of 40% is the most common one. If actual parenting time worked out to, say, 38% rather than 40%, section 9 does not apply at all, and the standard table amount based on the higher-earning parent's income applies in full, regardless of how much day-to-day involvement that parent otherwise has.

Parents sometimes assume a near-even schedule automatically means little or no support is owed. Because section 9 explicitly considers income disparity and increased costs, a genuinely shared 50/50 schedule between parents with very different incomes can still produce a meaningful support obligation from the higher earner.

Relying only on the written parenting plan, rather than the schedule actually followed, is another common problem. Courts look at what happened in practice, so a plan that calls for a 50/50 split on paper is not conclusive if one parent regularly ends up providing more of the actual care.

Finally, special or extraordinary expenses, covered separately at special and extraordinary expenses, are added on top of whatever section 9 amount is determined. They are not folded into the base calculation.

For a broader overview of how child support works across the country, including provincial variations, see Canada child support laws and Canadian law by province.

Disclaimer

This article provides general information about section 9 of the Federal Child Support Guidelines and is not legal advice. Child support outcomes depend on the specific facts of each family's situation, including the actual parenting schedule followed, both parents' incomes, and the children's needs. Anyone with a shared custody child support question should consult a family law lawyer or their provincial legal aid service.