One of the most common questions in Canadian family law is whether equal parenting time means no child support changes hands. The short answer: almost never. As long as the two parents earn different incomes, child support flows from the higher earner to the lower earner, even in a true 50/50 arrangement. The amount is reduced compared to sole custody, but it does not disappear.

This article explains how shared custody child support actually works in Canada: the 40% threshold that triggers section 9 of the Federal Child Support Guidelines (SOR/97-175), the three-factor analysis courts apply, and the straight set-off calculation that produces the monthly payment. We work through a concrete numerical example using the current federal child support tables and explain how Section 7 special expenses are handled separately.

The 40% Threshold — What It Actually Means

Shared custody, for child support purposes, is defined by a single bright-line rule: each parent must have the children for at least 40% of the time over the course of a year. This threshold is set out in section 9 of the Federal Child Support Guidelines. Courts typically measure parenting time by counting overnights, though in contested cases they have also considered waking hours or the proportion of a child's daily routine spent with each parent.

Below 40%, section 9 does not apply. The standard table amount under section 3 governs, and the full table amount for the payor's income is owed regardless of how close parenting time is to the threshold. At or above 40%, section 9 replaces the standard calculation with a more nuanced analysis.

Section 9 and the Three-Factor Analysis

When parenting time meets the 40% threshold, section 9 requires the court to consider three factors before determining the child support amount:

In the leading Supreme Court decision on shared custody support, Contino v. Leonelli-Contino (2005 SCC 63), the Court confirmed that all three factors must be considered and that no single factor should dominate the analysis. In practice, however, Canadian courts have largely converged on the "straight set-off" — the difference between the two table amounts under factor (a) — as the default starting point, departing from it only where factors (b) or (c) clearly warrant an adjustment.

How the Set-Off Is Calculated — A Worked Example

The straight set-off is straightforward arithmetic. Each parent's table amount is looked up as if the other had sole custody, and the higher earner pays the difference. Here is a concrete example using the current federal child support tables:

Parent A earns $85,000 and would owe $1,296 per month if Parent B had sole custody. Parent B earns $35,000 and would owe $534 per month if Parent A had sole custody. Because Parent A's notional obligation is higher, the net payment flows from Parent A to Parent B: $1,296 minus $534 equals $762 per month. The direction of the payment always runs from the higher-income parent to the lower-income parent, because the higher income produces the larger table amount.

Note that even in this 50/50 arrangement, $762 per month still flows between households. The set-off reduced the amount compared to sole custody (where Parent A would owe the full $1,296), but it did not eliminate it. The only scenario where 50/50 custody truly results in zero child support is when both parents earn exactly the same guideline income — producing identical table amounts and a set-off of $0.

What the Set-Off Does Not Cover

The set-off amount — base child support — is designed to cover the ordinary, recurring costs of raising children: food, clothing, shelter, basic school supplies, and routine activities that are part of everyday family life. It is not intended to cover large, discrete expenses that arise on top of those basics.

Those larger costs are governed by section 7 of the Guidelines, which deals with "special or extraordinary expenses." Section 7 covers categories such as childcare required for a parent's employment or education, uninsured health-related expenses (dental, orthodontic, therapeutic), post-secondary education costs, and extracurricular activities that are extraordinary given the family's circumstances. These are calculated and shared separately from the set-off — they are not absorbed into the base child support amount.

How Section 7 Expenses Are Split in Shared Custody

A common misconception is that shared custody changes how Section 7 expenses are divided. It does not. The s.7 apportionment formula is the same regardless of the parenting arrangement: each parent's share is proportional to their guideline income, adjusted for any spousal support being paid.

The formula works as follows. Each parent's guideline income is determined. If spousal support is in play, it is deducted from the payor's income and added to the recipient's. Each parent's share of the combined adjusted income becomes their percentage share of special expenses. For expenses that generate a tax benefit (such as childcare, which is deductible under ITA s.63), the expense is first reduced to its net cost after the tax benefit, and the net cost is then split proportionally.

Using our worked example above — Parent A earns $85,000 and Parent B earns $35,000, with no spousal support — the proportional shares are:

This means the higher-earning parent contributes through two channels: the net child support set-off and a disproportionate share of special expenses. For a deeper exploration of why the s.7 proportions and the NDI split never align — and whether they should — see our article on why your s.7 apportionment never matches your NDI split.

Imputed Income and Guideline Income in Shared Custody

The methodology for determining guideline income is the same regardless of whether the parenting arrangement is shared, split, or sole. Courts can impute income — assign an earning capacity higher than reported income — where a parent is voluntarily underemployed or unemployed, earning below what they could reasonably earn given their age, education, skills, and work history. The principles from Drygala v. Pauli (2002 ONCA) and related caselaw apply equally in shared custody cases.

Shared custody does not give either parent a basis to argue for a reduced income figure. Courts have rejected attempts to discount a primary residential parent's guideline income on the theory that they cannot work full-time while caring for children during their parenting time. Once guideline income is established — whether actual or imputed — the table lookup and set-off calculation are mechanical. Both parents' incomes receive equal scrutiny.

The "Cliff Effect" — Is Crossing 40% a Sudden Change?

The 40% threshold is a bright line in the statute, which creates a theoretical cliff: at 39.9% of parenting time, the full table amount applies; at 40%, the set-off analysis kicks in. In practice, the size of the cliff depends heavily on the income gap between the parents. For parents with similar incomes, the difference between the full table amount and the set-off is modest, so crossing the threshold changes the payment by a relatively small amount.

Where the cliff is real is in cases with large income disparities. If one parent earns $150,000 and the other earns $30,000, the gap between the full table amount under section 3 and the net set-off under section 9 can be several hundred dollars per month. A one-day-per-year change in the parenting schedule — tipping from 39% to 40% or vice versa — can swing the monthly obligation by $400 or more. Courts are aware of this effect and have discretion to smooth the transition in borderline cases, but that discretion is not always exercised.

When the Set-Off Can Be Adjusted

The straight set-off is the starting point, not the ceiling. Section 9(b) gives the court a basis to increase the support amount above the simple set-off where the increased costs of maintaining two full households for the children warrant it. This typically benefits the lower-income parent, who may struggle to provide an equivalent home environment on a reduced support payment. Courts have considered the cost of maintaining a second bedroom, duplicated essentials, and additional transportation expenses as evidence of increased costs under this factor.

Section 9(c) — the "conditions, means, needs, and other circumstances" factor — is a broad catch-all. It allows the court to adjust for economic hardship, disparity in the parents' standards of living, and any other circumstances that make the straight set-off inequitable. Undue hardship claims under section 10 of the Guidelines are also available in shared custody, though the bar is high: the claimant must show "undue" hardship, not merely that shared custody is expensive.

Finally, when spousal support is also in play, the child support and spousal support calculations interact. The SSAG with-child formula uses guideline incomes adjusted for spousal support to calculate each party's individual net disposable income (INDI), and the child support order feeds directly into that analysis. The two calculations are interdependent — changing one affects the other — which is why accurate software matters. Manual calculation of the interaction is prone to circular-dependency errors that compound across the child support, spousal support, and s.7 apportionment calculations.