For proceedings commenced in 2026, the Ontario prejudgment interest rate under section 128 of the Courts of Justice Act is 2.5%. It has held at 2.5% for every quarter published so far (Q1 through Q3). The rates are set quarterly by the Ministry of the Attorney General and published by the Government of Ontario; the Q4 rate is published closer to the quarter.

The quarter in which the proceeding was commenced, meaning the date the court issued the claim. It is not the quarter the cause of action arose, and it is not the rate in effect on the judgment date. That single rate then applies across the entire prejudgment period, even where the case takes years to resolve. This is the detail that trips people up most often, and it is why the calculator asks for the lawsuit date separately.

Ontario prejudgment interest is calculated under section 128 of the Courts of Justice Act. Interest runs from the date the cause of action arose to the date of the order, at the rate for the quarter the proceeding was commenced. The formula is: award amount × rate% ÷ 365 × number of days. This calculator applies that formula using the official quarterly rate tables going back to 1985.

It is simple interest, so it never builds on interest that has already accrued. Compounding only enters the picture where a contract pleaded in the claim calls for it, or in the narrow situations where a court uses its discretion under section 130 of the Courts of Justice Act.

Section 128(4) keeps several categories out of the calculation. As a general rule, you cannot claim prejudgment interest on punitive or exemplary damages, on interest itself, on costs of the proceeding, on the slice of an award reflecting pecuniary loss that arises after the order date, or on amounts already carrying interest as of right, such as under a contract that sets its own rate.

No. Under subsection 128(2) of the Courts of Justice Act, the statutory prejudgment interest rate does not apply to non-pecuniary loss in personal injury actions. The rate for that head of damages is set by rule 53.10 of the Rules of Civil Procedure, which is updated quarterly by the Civil Rules Committee. This Ontario prejudgment interest calculator should not be used for non-pecuniary damages in personal injury cases.

Once the court has made its order, a different clock starts. Postjudgment interest under section 129 runs on the amount ordered plus costs plus the prejudgment interest, at a separate (typically higher) quarterly rate, until the judgment is paid. Use our free Ontario Postjudgment Interest Calculator for that calculation, including partial payments.

Yes. The calculator is completely free, with no account, login, or payment required. It is provided by Courtready as a public legal education resource. It is a reference tool only and does not constitute legal advice. Always verify the applicable rate against the Government of Ontario published rate tables and the current version of the Courts of Justice Act.