Ontario Court Interest Calculator
Result - Prejudgment interest
Formula: Interest = Principal × Rate × (Days ÷ 365). Simple interest, calculated on the outstanding principal only.
Enter the amount, the annual rate and the two dates above to see the calculation, the period breakdown and the copy and print options.
About this calculator
In Ontario, interest on money judgments is governed by sections 127–130 of the Courts of Justice Act. Prejudgment interest generally runs to the date of the order at a rate tied to the quarter in which the proceeding was commenced, subject to statutory exceptions and to the court's discretion. Postjudgment interest generally runs from the order date at a rate tied to the quarter of the order.
Section 127 is the definitions provision: it defines the prejudgment and postjudgment interest rates and the dates by reference to which they are set. The interest itself is awarded under sections 128 and 129. This calculator follows the ordinary simple-interest methodology used for those sections and reflected in the Ministry's published calculation guidance: interest on the outstanding principal, never added to the balance it is charged on. There is no compounding mode. A statute, a contract or the terms of the order can displace that ordinary position, and the court may adjust or refuse interest under sections 128, 129 or 130.
This tool computes simple interest on the exact number of calendar days between the dates you provide, using a 365-day denominator. Rate lookups and legal characterisation (for example, whether a claim is for non-pecuniary damages) should be confirmed against the statute and the Ministry's published rate table.
Worked example
Three worked examples, each exercising a different behaviour of the calculation. All figures are illustrative arithmetic on assumed inputs, not statements of entitlement.
Example 1 - a part payment mid-period
Facts. An unpaid amount of $8,000 runs at 2.5% per year simple for 60 days. The debtor then pays $3,000, and the reduced balance of $5,000 runs at the same rate for a further 60 days.
First period. $8,000 × 2.5% ÷ 365 × 60 days = $32.88.
Second period. $5,000 × 2.5% ÷ 365 × 60 days = $20.55.
Interest across both periods. The tool keeps each period's interest unrounded, adds them, and rounds only for display, so the combined figure is $53.42 on a closing balance of $5,000 in principal. Adding the two displayed period amounts by hand will usually match, but it can differ by a cent because each displayed amount has already been rounded. Where a figure has to reconcile exactly with someone else's ledger, say which convention you used.
The payment does not restart interest. It ends one period and opens another at the lower balance. Whether the $3,000 reduces principal first or accrued interest first depends on the order, agreement or enforcement rule that applies, which is why the tool asks you to choose.
Example 2 - the allocation choice changes the answer
Facts. A judgment of $20,000 made on 1 March 2024 carries postjudgment interest at 7.0%, the rate published for the quarter in which that order was made. Nothing is paid until 1 March 2026, when the debtor pays $5,000. The creditor calculates the balance as at 1 September 2026.
Interest to the payment date. 1 March 2024 to 1 March 2026 is 730 days: two consecutive 365-day anniversary-to-anniversary periods (1 March 2024 to 1 March 2025, then 1 March 2025 to 1 March 2026). Neither period contains 29 February, so this two-year span contains no leap day: $20,000 × 7.0% ÷ 365 × 730 = $2,800.00.
Interest-first. The $5,000 clears the $2,800.00 of accrued interest and the remaining $2,200.00 reduces principal to $17,800.00. Interest for the next 184 days is $17,800 × 7.0% ÷ 365 × 184 = $628.12, for a balance of $18,428.12.
Principal-first. The whole $5,000 reduces principal to $15,000.00, leaving $2,800.00 of interest still outstanding. Interest for the next 184 days runs on the lower principal: $15,000 × 7.0% ÷ 365 × 184 = $529.32, for a balance of $18,329.32.
Same money, same dates, a difference of about $98.80 after six months - and it widens the longer the balance runs. The allocation is a modelling assumption you select, not a legal conclusion; check the order, the agreement, or the applicable enforcement rule.
Example 3 - a payment larger than the balance, and why the rate must match the stage
Facts. $1,000 runs at 5.0% from 1 January 2026. On 1 July 2026 (181 days later) the debtor pays $1,100 in a single transfer.
Interest to the payment date. $1,000 × 5.0% ÷ 365 × 181 = $24.79. The amount required to clear the debt that day is $1,024.79.
What the ledger shows. $1,024.79 is credited, the balance goes to nil, and the excess $75.21 is reported separately as an overpayment rather than driving the balance negative. Nothing accrues after that date.
The tool does not decide what happens to the surplus - refund, credit against costs, or something else. Note also that if this amount had moved from prejudgment to postjudgment status partway through, you would run two calculations, each at the rate for its own stage, rather than one continuous period at a single rate.
How this calculation works
Every period in this tool uses one simple-interest formula: unpaid amount × annual rate ÷ 365 × the actual number of days in the period. Nothing compounds. Interest is never added to the balance that interest is then charged on, so a longer period earns no more per day than a short one at the same balance.
The rate is not something the tool knows. You supply it, and which rate is the right one depends on the stage of the proceeding. For prejudgment interest the statutory rate is selected by the quarter in which the proceeding was commenced, and the period ordinarily runs from the date the cause of action arose to the date of the order. For postjudgment interest the statutory rate is selected by the quarter in which the order was made, and the period ordinarily runs from the date of the order until payment. The postjudgment base may include the judgment amount, costs awarded and prejudgment interest already included in the order.
A contract, another statute or the terms of the order itself may displace the statutory rate. Where that happens, use the custom mode and enter the rate that actually governs.
| Input | How the tool treats it | What it does not do |
|---|---|---|
| Unpaid amount | Becomes the balance for the first period. | Does not test whether the amount is owed or correctly quantified. |
| Annual rate | Divided by 365 and applied per day, unchanged for the whole period. | Does not look up the quarterly statutory rate for you. |
| Start and end dates | Counted as actual calendar days, including leap days. | Does not decide when the cause of action arose or when the order took effect. |
| Payment | Closes the current period on its date and opens a new one at the reduced balance. | Does not decide the legally correct allocation between interest and principal. |
| Allocation choice | Applies each payment to interest first or principal first, as selected. | Does not verify the choice against the order, agreement or enforcement rule. |
What the tool includes
- Simple interest at a single rate you supply, over the exact days between the dates you enter.
- Separate prejudgment, postjudgment and custom modes, so the rate you enter is tied to the stage you are calculating.
- A payment ledger: any number of dated part payments, each splitting the timeline into its own period.
- A choice between interest-first and principal-first allocation, with the resulting balance shown period by period.
- A per-period breakdown showing days, interest and closing balance, so every figure can be checked by hand.
What it does not decide
This is an arithmetic tool. It does not decide entitlement, and its output is never a statement of what a court will award. The following situations are not supported and need legal review before any figure is used:
- Past pecuniary loss that must be calculated in six-month segments rather than as one continuous period.
- Non-pecuniary loss in personal-injury claims, which is treated differently for prejudgment interest.
- Punitive, aggravated or exemplary damages.
- Consent orders and settlements where the parties have agreed their own interest terms.
- Periodic payment orders and structured awards.
- Foreign orders, foreign currency amounts and orders registered from another jurisdiction.
- Any case where the court has varied, disallowed or extended interest under its discretion in section 130 of the Courts of Justice Act.
The tool also does not choose your rate, confirm the correct start date, or decide whether costs and prejudgment interest form part of the amount that carries postjudgment interest.
Common mistakes
- Using today's quarterly rate for a proceeding commenced years ago. The prejudgment rate is fixed by the quarter of commencement, not the quarter you are doing the maths in.
- Using the prejudgment rate for the postjudgment period. They are selected by different quarters and are frequently different numbers.
- Compounding. Adding each year's interest to the balance overstates the total; the ordinary statutory calculation under sections 128 and 129 of the Courts of Justice Act is simple interest, and this tool has no compounding mode.
- Entering one long period and a single lump-sum payment at the end. Each part payment must be dated, or the balance is overstated for the days after it was received.
- Forgetting that the postjudgment base may include awarded costs and prejudgment interest included in the order, not only the damages figure.
- Treating a 360-day or monthly approximation as equivalent. This tool divides by 365 and counts actual days, so the two will not reconcile.
Before relying on the result
- Confirm the rate against the Ministry of the Attorney General's quarterly table for the correct quarter, and read the table's own notes.
- Confirm the start date: the date the cause of action arose for prejudgment interest, and the date of the order for postjudgment interest.
- Check the order itself for a rate, an interest start date or a direction that displaces the statutory position.
- Check whether the claim falls into one of the unsupported categories listed above.
- Re-run the calculation with the opposite allocation assumption to see how much the answer moves.
- Take legal advice before serving, filing or paying on any figure produced here.
Legal information reviewed: 12 September 2026. Checked against the official sources listed on this page. Rates, fees and rules change; confirm anything you rely on.
Fixed rules and your assumptions
Some of what appears in the result is fixed by Ontario law and does not change from case to case. The rest is an assumption you have chosen, and a different assumption produces a different number. Knowing which is which is the difference between a figure you can defend and a figure you cannot.
| Element | Status | Where it comes from |
|---|---|---|
| Simple, non-compounding interest | Fixed rule | The ordinary methodology for interest awarded under sections 128 and 129 of the Courts of Justice Act, unless a statute, agreement or order provides otherwise. |
| 365-day denominator and actual day count | Fixed method in this tool | Standard treatment for the statutory calculation; a contract using a different convention must be calculated on its own terms. |
| Which quarterly rate governs | Fixed rule, applied by you | Section 128 or 129 selects the quarter; you read the rate off the Ministry table and enter it. |
| Interest start date | Your input | Depends on when the cause of action arose, or on the date of the order. |
| Payment allocation | Your assumption | The order, the agreement or the enforcement rule governs; the tool applies whichever you select. |
| The base amount | Your input | Whether costs and prejudgment interest form part of the amount carrying postjudgment interest depends on the order. |
| Whether interest is owed at all | Not decided here | Entitlement is a legal question; section 130 also lets the court vary or refuse interest. |
Practical next steps
- Print or copy the period-by-period breakdown and keep it with the file, so the figure can be reproduced later.
- Record the exact date the calculation runs to. A payout figure is only good for that day, and should be quoted with the date attached.
- If you are claiming interest in a proceeding, confirm it is actually pleaded, and state the statutory or contractual basis you rely on.
- If the matter has moved from prejudgment to postjudgment, run two calculations at the rates for their own stages rather than one continuous period.
- Re-run with the opposite allocation assumption and note the spread, so you know how sensitive the number is before negotiating on it.
- Read the companion guides for the legal and procedural context, then take advice before filing, serving, enforcing or paying on any figure.
Source versions and revision note
The legal statements on this page were checked on 21 August 2026 against the consolidated Courts of Justice Act on Ontario's e-Laws service (sections 127-130) and against the Ministry of the Attorney General's published quarterly prejudgment and postjudgment interest rate table, both linked below. The rate table is republished each quarter, so the version you consult may be newer than the version checked here; always read the current table for the quarter that governs your matter.
At that review the introduction, the worked examples, the fixed-rules table and the next-steps section were added or expanded. No change was made to the calculation engine: it continues to compute simple interest on actual calendar days over a 365-day year, with payments credited on their own dates.
Correction note, 12 September 2026. The built-in example rates were corrected (the prejudgment example for a proceeding commenced on 10 February 2023 now uses 4.0%, and the postjudgment example for an order made on 1 March 2024 now uses 7.0%), and the dependent figures were recalculated from the engine. Statements attributing the simple-interest rule to section 127 were corrected: section 127 defines the rates and the dates by reference to which they are set, and the ordinary simple-interest methodology applies to interest awarded under sections 128 and 129. Payment allocation is now described consistently as this tool's default modelling assumption rather than a legal conclusion. Input validation was tightened so an incomplete or impossible entry blocks the result instead of being silently dropped, and the copy output was expanded. The calculation engine and the published date are unchanged. This is an editorial correction, not a certified or legal review.
Frequently asked questions
Which rate should I enter?
Why simple interest?
How are payments applied?
Related calculator and guides
- Ontario prejudgment interest, explained
Where the rate comes from, when interest starts running, and why the court can vary or refuse it.
- Ontario postjudgment interest, explained
How the rate is fixed at the date of the order and how partial payments are applied.