Child Support in Shared Custody: How the 40% Rule Works

Many parents assume that if the children are with each of them half the time, nobody pays anyone. It is an understandable assumption, and it is usually wrong. Shared parenting changes how child support is calculated, not whether it exists. Knowing how the calculation works can take a lot of heat out of the conversation, because the question stops being "who wins" and becomes "what does the formula say".

What does "shared custody" mean legally?

Since 2021, the Divorce Act no longer uses the word "custody". It talks about decision-making responsibility and parenting time. For child support, what matters is parenting time. The threshold in the Guidelines is at least 40 percent of parenting time with each parent over the course of a year.

Forty percent works out to roughly 146 days a year, or about 5.6 days in every 14. Courts look at the whole year, so holidays, summer schedules and school breaks count, not only a typical school week.

Overnights are the most common way to count because they are easy to document, but courts can look at time and responsibility more broadly, for example a parent who has the children every day after school until bedtime.

Reaching 40 percent does not automatically reduce support. It changes which section of the Guidelines applies.

Under 40 percent: the standard table applies

If one parent has the children more than 60 percent of the time, the calculation is simple. The other parent pays the table amount for their income, province and number of children, plus their share of Section 7 expenses. Our guide to how child support works in Canada explains the tables.

At or above 40 percent: Section 9 applies

When each parent has at least 40 percent, Section 9 says the amount must be determined by taking into account three things:

1. The table amounts for each parent, meaning what each would pay the other if the other had the children most of the time.
2. The increased costs of shared parenting. Two child-ready homes cost more than one: two bedrooms, two sets of clothes, two sets of school supplies.
3. The conditions, means, needs and other circumstances of each parent and the children. This is the real economic picture, not just the formula.

How does the set-off approach work in practice?

The usual starting point is a set-off: work out each parent's table amount as if they were the payor, then subtract the smaller from the larger. The higher earner pays the difference.

The set-off is not the end of the analysis. In Contino v. Leonelli-Contino (2005), the Supreme Court of Canada said the simple set-off is a starting point, not a presumption. Courts can look at each household's budget, who actually pays for clothing and activities, and whether the result would leave the children with a very different standard of living in each home.

Section 7 expenses don't change

Section 7 expenses, such as childcare, health costs over insurance, extraordinary extracurriculars and post-secondary costs, are still shared in proportion to income, regardless of how parenting time is split.

Using the example above, combined income is $160,000. Parent A pays 62.5 percent of agreed Section 7 costs and Parent B pays 37.5 percent. This is on top of the Section 9 amount.

Parenting time and money: keep them separate

Because 40 percent can change the number, it is tempting to push for more time partly to pay less. Courts and mediators see this often. Parenting time is decided on the child's best interests, and a schedule that looks designed around the support calculation tends to be viewed with suspicion.

The opposite is also true. A parent who resists a reasonable shared schedule mainly to protect support is not acting in the child's interests either. If you notice money shaping your view of the schedule, it is worth naming that, privately, and setting it aside. A mediator can help both of you do this.

When parenting time shifts over time

A teenager's schedule looks nothing like a toddler's. Children change schools, take jobs, or start spending more time in one home. When the real pattern moves above or below 40 percent, support should be revisited.

Under Section 25 of the Guidelines, the recipient under an order can ask in writing, once a year, for the payor's income documents. A good agreement goes further: both parents exchange tax documents every year by a set date and recalculate, and either can ask for a review if the schedule changes for more than a few months. If a change is ignored for years, a court can order retroactive adjustments.

How does shared custody affect child support in the US?

Each state sets its own child support guidelines, and most give some kind of adjustment for shared parenting, but the threshold and the method vary. Some states use a number of overnights per year; others use a percentage of time; some apply a cross-credit similar to Canada's set-off. Check your state's child support agency worksheet or start with your state hub.

What to do this week

If the other parent is disputing how much time the children actually spend with you, or one household is struggling badly under the set-off, get advice from a family lawyer or mediator before you sign. Section 9 leaves room for judgement, and that is where advice pays for itself.

Common questions

If we split time 50/50, does anyone pay child support?

Usually yes, if incomes differ. The typical starting point is to set off the two table amounts, so the higher earner pays the difference, then adjust for actual costs.

How is 40 percent counted?

Over a full year, including holidays and school breaks. Overnights are the most common measure, which works out to about 146 a year, but courts can consider time and responsibility more broadly.

Do we still split Section 7 expenses in shared parenting?

Yes. Special and extraordinary expenses are shared in proportion to income no matter how parenting time is divided.

What happens if the schedule changes after we sign?

If parenting time moves above or below 40 percent for a sustained period, support should be recalculated. A review clause in your agreement makes this simpler and avoids retroactive claims.

This guide is legal information, not legal advice, and it is written for a general audience across Canada and the United States. Family law is provincial and state-based and changes over time. Before you rely on anything here for your own situation, confirm it with a qualified family lawyer in your province or state. FairWell can connect you with one through the professional directory.