Shared custody does not eliminate child support. It changes how it's calculated. The 40% threshold, Section 9, the set-off approach, and what to nail down in your agreement.
One of the most common questions separating parents have is whether child support changes when both parents share parenting time roughly equally. The short answer: yes, but not in the way most people expect.
Shared custody does not eliminate child support. It changes how it's calculated.
In Canadian family law, the threshold for shared parenting that triggers a different child support calculation is 40% of time with each parent over the course of a year.
40% works out to roughly 146 nights per year, or about 6 nights in every two-week period. Courts look at time across the whole year, not just typical weeks, so holiday arrangements, summer schedules, and school breaks all count.
Courts measure time with the child, not just overnight stays, though overnights are the most commonly used measure in practice because they're clearest to document.
Meeting the 40% threshold doesn't automatically reduce support. It changes which section of the Federal Child Support Guidelines applies to the calculation.
When one parent has primary care, meaning the child is with the other parent less than 40% of the time, the calculation is straightforward. The parent who spends less time with the child pays the table amount from Schedule I of the Federal Child Support Guidelines, based on their gross income and the number of children. That's it.
When each parent has the child at least 40% of the time, Section 9 of the Guidelines kicks in. Section 9 allows courts to depart from the straight table amount and consider three factors:
The most common method under Section 9 is the set-off: calculate what each parent would pay under the table if they were the payor, then subtract the smaller from the larger. The higher-earning parent pays the difference to the lower-earning parent.
Parent A earns $100,000. Table amount: $1,383/month.
Parent B earns $60,000. Table amount: $831/month.
Set-off: $1,383 − $831 = $552/month payable from Parent A to Parent B.
The set-off isn't automatic, courts can adjust it based on the Section 9 factors, but it's the most frequently used starting point.
Note that the higher-earning parent almost always still pays something. Shared time does not produce a zero outcome when there's a meaningful income difference between the parents.
Section 7 expenses, childcare, medical costs, extraordinary extracurriculars, post-secondary education, are calculated and shared proportionally regardless of the parenting time arrangement. The proportional split is based on each parent's income relative to combined income, not on how many nights each parent has.
If the lower-earning parent earns $60,000 and the higher-earning parent earns $100,000, combined income is $160,000. The higher-earning parent pays 62.5% of shared Section 7 costs; the lower-earning parent pays 37.5%.
This is on top of whatever base support amount is established under Section 9.
One issue that arises in shared custody cases is the incentive to argue for more parenting time in order to reduce child support. Courts are aware of this dynamic. They look at whether a custody arrangement genuinely serves the child's interests, not just a parent's financial interests.
If parenting time appears structured primarily to reduce support obligations rather than to serve the child, courts are unlikely to reward it with a reduced support order. The child's best interests remain the governing standard, separate from the financial dispute.
Children's needs and living arrangements change. A teenager's schedule looks nothing like a toddler's. As parenting time evolves, gradually or as a result of the child's own preferences, child support should be revisited.
Both parents are required under Section 25 of the Federal Child Support Guidelines to disclose their income annually. When parenting time changes materially, that's also a trigger for reviewing the support amount.
Courts can order retroactive support adjustments in some circumstances, so ignoring a meaningful change in parenting time or income for an extended period creates exposure.
Before any conversation about whether shared custody applies, count the actual nights per year. Include holidays, school breaks, and typical weeks. Build a calendar and add it up. 40% matters, don't assume you're there without checking.
Whatever base support amount is established, the treatment of Section 7 expenses needs to be clear in your separation agreement. Vague language about "sharing expenses" produces endless disputes. Name the categories, set the split, establish how costs get submitted and reimbursed.
Support set up when children are young needs to be revisited as schedules shift and incomes change. Build a review mechanism into your agreement.
FairWell's calculator handles both the standard table calculation and the Section 9 shared-custody scenario. Enter both parents' incomes and the custody arrangement, and it walks you through the set-off calculation and Section 7 proportional split. It's designed to give you an accurate, defensible number before any negotiation starts.
This guide is for informational purposes only. FairWell is not a law firm and this is not legal advice. Every situation is different, consult a qualified family lawyer in your province or state.
FairWell's calculator handles the Section 9 set-off, Section 7 proportional split, and both parents' incomes in one place.
Use the calculator →Child support calculations depend on specific facts and jurisdiction. Before finalising any arrangement, speak with a qualified family lawyer in your province.