Child Support Calculator Ontario: Your Number — and the Math Behind It

Ask a family lawyer how child support is calculated and you’ll usually hear: “We have the Child Support Tables.” True — but the tables are not the formula. They’re the output of a formula, and almost nobody who pays or receives child support is ever shown how that formula works.

I’ve sat across from hundreds of separating parents, and I’ve watched what happens when a number arrives with no explanation. It feels arbitrary. It feels imposed. And a support amount that feels imposed gets resented, disputed, and — too often — unpaid.

When I take twenty minutes in mediation to walk both parents through the actual math, something changes: the number stops being a verdict and starts being arithmetic. Parents who understand their number argue about it far less.

That’s what this page is for. You’ll get your number from the calculator above. The rest of the page shows you where it comes from — and what the calculator can’t tell you.

The Federal Child Support Guidelines were introduced in 1997 to fix a real problem: before them, support awards were inconsistent, unpredictable, and often too low. The Guidelines replaced judicial guesswork with a standardized formula, so that parents in similar circumstances pay similar amounts anywhere in the province. The formula behind the tables aims to balance living standards between the two households after separation.

It works from a handful of core assumptions:

• Income drives everything. The paying parent’s gross annual income — line 15000 of the tax return — is the engine of the calculation. Taxes and payroll deductions are modelled inside the formula; you don’t subtract them yourself.

• Costs scale with household size, not linearly. The formula uses a 40/30 “equivalence scale”: if a single adult’s living costs equal 1.0, a first child adds roughly 40% and each additional child adds roughly 30%. A parent with one child is a 1.4-adult-equivalent household; with two children, 1.7.

• The receiving household is assumed to carry the day-to-day costs. The original model assumes the parent the children primarily live with pays the child-related expenses, and the paying parent contributes through support.

That equivalence scale is the piece nobody explains, and it’s the piece that makes the table amounts make sense. The tables aren’t a percentage of income pulled from the air — they’re the amount that, after modelled taxes, moves the two households toward comparable standards of living given those adult-equivalent weights.

Is the model perfect? No — and I’ll show you its biggest blind spot further down. But it isn’t arbitrary either, and knowing the machinery is the first step to trusting (or fairly challenging) your number.

Here is the calculation the tool above performs, in the order the Guidelines prescribe:

1. Confirm which framework applies. Married and divorcing parents fall under the federal Divorce Act; never-married and common-law parents fall under Ontario’s Family Law Act. It rarely matters to the dollar amount — Ontario’s regulation mirrors the federal tables exactly.

2. Count the children entitled to support. Children under 18, plus adult children who can’t be self-supporting because of illness, disability, or full-time studies.

3. Establish the parenting arrangement. If the children live primarily (more than 60% of the time) with one parent, the other parent pays the table amount. If parenting time is shared more evenly, the math changes — see below.

4. Determine gross annual income. Line 15000 for employees. For self-employment, corporate, rental, or variable income, this step is where most disputes live.

5. Look up the table amount for that income, that number of children, in the Ontario table.

6. Add section 7 special or extraordinary expenses — child care, medical and dental costs beyond insurance, post-secondary education, and extracurriculars that are necessary and reasonable. These are shared in proportion to both parents’ incomes, on top of the table amount. Full breakdown here: [Section 7 Expenses](https://divorcethesmartway.ca/support/section-7-expenses/).

7. Check for undue hardship. A narrow escape valve — a legal duty to support another child, unusually high access costs — available only if the claiming parent’s household standard of living is lower than the other’s.

Take a parent earning $80,000 with two children living primarily with the other parent. The Ontario table amount is approximately $1,220 per month — VERIFY. Now suppose the children’s daycare costs $600 per month after subsidies and tax credits, and the parents’ incomes split 60/40. The paying parent adds 60% of $600 — $360 — to the table amount, for a total of about $1,580 per month. Notice what drove that number: income, child count, and one section 7 expense. Not fault. Not who left. Not what either parent thinks the other deserves. That’s the design.

The tables were replaced on October 1, 2025 — the first full revision since 2017. The formula didn’t change; it was re-run against current federal and provincial tax rules. Three things every Ontario parent should know:

• The floor moved to $16,000. A parent earning $16,000 or less in gross annual income now has a table amount of $0, reflecting the updated basic personal amount at which income tax begins.

• Lower-middle incomes generally pay less. Between roughly $16,000 and $45,000, the new amounts commonly came down from the 2017 figures. Above $45,000, most one- and two-child amounts moved only 1–2% in either direction — VERIFY against the current tables for your income.

• Existing orders did not update automatically. The new tables apply to orders and agreements made after October 1, 2025. If your order predates that and the new table amount differs from what you pay or receive, that gap is grounds to seek a change — through the Ontario Child Support Service online or a Motion to Change — not an automatic adjustment.

If you ran a calculator before October 2025, run it again. And if a website tells you the tables “haven’t changed since 2017,” it’s out of date