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How this works
This calculator adds up what the table says should have been paid for each past month, then subtracts what was actually paid. It uses the table that was in force at the time: the 2017 tables for months up to September 2025 and the 2025 tables from October 2025 on.
A worked example
A parent in Ontario earning $70,000 paid nothing for one child from April 2025 to March 2026.
- April to September 2025 is 6 months at $654 under the 2017 table: $3,924.
- October 2025 to March 2026 is 6 months at $648 under the 2025 table: $3,888.
- The total is $7,812.
What a court looks at
The table gives the arithmetic. A court decides how far back to go. Under the Supreme Court of Canada’s decision in D.B.S. v. S.R.G., the usual start date is when the other parent was first asked for support or for updated income, and generally no more than three years before a formal claim. A court can go back further when the paying parent hid income. Read the full guide to back support and arrears.
If the paying parent’s income changed from year to year, run each year separately and add the results. Special expenses are not included.
Common questions
How do I calculate back child support in Canada?
For each month, take the table amount for the paying parent’s income, province and number of children under the table in force that month, and subtract what was paid. The 2017 tables apply up to September 2025 and the 2025 tables after that.
How far back can back child support go?
Generally to the date the other parent was first asked for support or for updated income, and usually no more than three years before a formal claim. A court can go back further if income was hidden.
What is the difference between arrears and retroactive support?
Arrears are amounts an order required that were not paid. Retroactive support covers a time when no order existed or the amount was based on an outdated income.
