How to Calculate Post-Judgment Interest in Ontario

Why Post-Judgment Interest Matters for Enforcement

You won your Ontario Small Claims Court case. The judge awarded you money. But every day the debtor doesn't pay, your judgment grows — because post-judgment interest accrues automatically under section 129 of the Courts of Justice Act (CJA).

Here's the problem: when you file an Affidavit for Enforcement Request (Form 20P) to start garnishment or a writ of seizure and sale, the court clerk expects an accurate interest calculation. Get the math wrong and your filing gets rejected at the counter. That's wasted time and, if you have to refile, potentially wasted fees.

The formula itself is straightforward. The tricky part is keeping it accurate as partial payments come in and quarterly rates change.

The Formula: Simple Interest, Not Compound

Ontario post-judgment interest uses simple interest calculated daily:

Interest = Principal × Rate × Days ÷ 365

The "principal" for post-judgment interest is the total awarded amount — that includes the judgment principal, any costs the judge awarded, and any prejudgment interest that was part of the order.

The "rate" comes from the quarterly post-judgment interest rate published by the Ontario Ministry of the Attorney General under CJA section 129. For Q4 2026, that rate is 4.0% per annum. The prejudgment default rate for the same quarter is 2.5%.

These rates change every quarter. You need to check the published rate for each quarter your judgment spans and calculate each period separately if the rate shifts.

A Worked Example

Say you have a judgment for $8,500 in principal plus $1,275 in costs, entered on March 15, 2026. No payments received. You want to calculate the total owing as of October 17, 2026.

Total awarded amount: $8,500 + $1,275 = $9,775

Assuming a flat 4.0% rate across that period (check actual published rates for each quarter):

• March 15 to October 17 = 216 days
• Interest = $9,775 × 0.04 × 216 ÷ 365 = $231.38

Total owing: $9,775 + $231.38 = $10,006.38

That's the figure you'd enter on Form 20P when requesting enforcement.

How Partial Payments Change the Calculation

When a debtor makes partial payments — through garnishment, voluntary payments, or sheriff seizure — the payment allocation follows a strict order:

1. Court disbursements first — filing fees and service costs you incurred during enforcement
2. Accrued post-judgment interest — outstanding interest to the date of payment
3. Principal — only after interest and costs are satisfied

This means partial payments reduce the principal slowly. You need to recalculate interest on the reduced principal from each payment date forward. A running ledger is the only way to keep this accurate over months of garnishment installments.

Where the Rate Comes From

The post-judgment interest rate under CJA section 129 is tied to the federal bank rate and published quarterly by the Ministry of the Attorney General. It's the same rate for all civil judgments in Ontario — Small Claims Court included.

You can find the current rate on the Ontario government's published schedule. The rate applies from the date of judgment until full payment, changing at the start of each calendar quarter if the published rate moves.

One common mistake: confusing the prejudgment rate (CJA section 128) with the post-judgment rate (section 129). They're published together but they're different numbers. The prejudgment rate applies to the period before judgment; the post-judgment rate applies after. If your judgment order specifies a contractual interest rate, that overrides the statutory rate — but only if the judge explicitly ordered it.

When You Need This Calculation

You'll calculate post-judgment interest every time you:

• File Form 20P (Affidavit for Enforcement Request) to start any enforcement step
• File Form 20E (Notice of Garnishment) and need to state the total amount owing
• Update the court on remaining balance after partial payments
• Determine whether the full judgment has been satisfied so you can terminate enforcement

The court clerk does not calculate your interest for you. They check that you've shown your work and that the math is reasonable. Filing with an inflated or incorrect figure can delay your enforcement or, worse, get your affidavit returned.

Getting the Numbers Right on Form 20P

Form 20P requires you to state: the original judgment amount, costs awarded, post-judgment interest accrued, payments received, and the balance owing. Each figure needs a date reference so the clerk can verify the calculation.

The cleanest approach is maintaining a simple ledger from the day your judgment is entered. Record: the judgment date and total awarded, each quarterly rate period, each payment received (with date and allocation), and a running balance. When it's time to file Form 20P, you pull the current figures directly from the ledger.

If you're enforcing a judgment that spans multiple quarters with different rates, you'll need to calculate interest separately for each rate period and add them together. This is where most self-represented creditors make errors — they use a single rate for the entire period instead of splitting at quarterly boundaries.

Our Ontario judgment enforcement guide includes a post-judgment interest ledger worksheet that handles rate changes and partial payment allocation automatically. It's built specifically for the math you need on Form 20P.

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