Worked example: interest in Canada

A late payment on a Canadian invoice or judgment can quietly grow into a much larger number. This worked example walks through how interest can play out in a realistic Canadian scenario, and how to explore “what ifs” using the DocketMath interest calculator.

Throughout, the focus is on the mechanics of the calculation, not on what rate you should use or what you’re entitled to—that’s a legal question and depends on your contract and applicable law.

Example inputs

We’ll use a concrete, business‑style scenario involving a late commercial invoice in Ontario. You can replicate this with the DocketMath interest calculator.

Here is a simple illustration for Canada. These values are for demonstration only and should be replaced with your actual inputs.

• Principal or amount: $120,000
• Rate or cap: 12%
• Start date: 2025-01-15
• End/as-of date: 2025-09-30

Scenario

• A supplier issues an invoice for CAD 25,000 to a customer in Toronto.
• The contract says:

  • “Interest at 12% per annum, calculated monthly, on all overdue amounts.”
• The customer doesn’t pay on time.
• The supplier wants to know how much interest has accrued by the time payment finally arrives.

Key assumptions for the example

We’ll keep the setup simple and explicit.

Principal and dates

• Principal (amount owing): $25,000
• Invoice due date: March 1, 2023
• Interest start date: March 2, 2023
  (first day the amount is overdue)
• Payment date (calculation end date): September 15, 2024

Interest terms

• Annual interest rate: 12%
• Compounding: Monthly
• Interest basis: Simple day count between compounding dates
  (i.e., interest accrues daily but is added to principal monthly)
• Currency: CAD

Jurisdiction

• Jurisdiction: Canada (Ontario)
  (This matters if you’re comparing against statutory or court rates, but here we’re just applying a contract rate.)

How the calculator inputs map to the scenario

If you open DocketMath’s interest tool, the fields would roughly look like:

• Principal amount: 25000
• Start date: 2023‑03‑02
• End date: 2024‑09‑15
• Annual rate: 12%
• Compounding: Monthly
• Jurisdiction: Canada (or province, depending on the UI)
• Basis / method:

  • “Compound interest”
  • Day‑count: “Actual/365” or similar (many Canadian contexts use an actual‑days basis; always check your specific rules or contract wording.)

Example run

Let’s walk through what the DocketMath interest calculator is effectively doing behind the scenes.

We’ll assume:

• Annual rate: 12%
• Monthly rate: 12% ÷ 12 = 1% per month
• Day count: actual days in each period, with interest allocated proportionally for partial months.

1. Overall time period

From March 2, 2023 to September 15, 2024:

• 2023‑03‑02 to 2023‑12‑31: 305 days
• 2024‑01‑01 to 2024‑09‑15: 258 days
• Total: 563 days (about 18.5 months)

Interest compounds monthly, so we split this into:

• Full months where interest is added to principal
• A final partial month

2. Month‑by‑month sketch

This is a conceptual breakdown; DocketMath does the exact day arithmetic for you.

We’ll show a simplified table with selected checkpoints. (Numbers rounded for clarity.)

*In practice, the calculator uses exact day counts, not approximate month lengths.

This shows that:

• Over roughly 18.5 months, the balance grows from $25,000 to about $30,001.
• That’s about $5,001 in interest.

You can think of it in two layers.

1. Simple interest baseline

   If we ignored compounding and just did simple interest:

   • 12% per year on $25,000 = $3,000 per year
   • 563 days is about 1.54 years (563 ÷ 365)
   • Simple interest ≈ $3,000 × 1.54 ≈ $4,620

2. Effect of monthly compounding

   Because interest is added to principal every month, you also earn interest on prior interest. That extra:

   • ≈ $5,001 (compound) − $4,620 (simple) ≈ $381

   is the additional cost of compounding over this period.

The calculator essentially automates:

1. Splitting the total period into compounding intervals.
2. Applying the periodic rate (1% per full month).
3. Handling the final partial month with a pro‑rated rate:

   • Daily rate ≈ 12% ÷ 365 ≈ 0.03288% per day
   • 14 days ≈ 0.4603%
   • Interest for last 14 days ≈ $29,864 × 0.4603% ≈ $137

3. What you’d see in DocketMath

A typical output summary for this run (rounded) would look like:

• Principal: $25,000.00
• Start date: 2023‑03‑02
• End date: 2024‑09‑15
• Annual rate: 12%, compounded monthly
• Total interest: ≈ $5,001.00
• Total amount due: ≈ $30,001.00

Using the explain‑style breakdown (where available), you can also see:

• Each compounding date
• Interest added at each step
• The evolving principal balance

This is especially useful when sanity‑checking interest claims in a demand letter, pleading, or affidavit.

Sensitivity check

The real power of a calculator like DocketMath is testing how sensitive the result is to:

• Rate changes
• Compounding frequency
• Time (earlier or later payment)
• Simple vs. compound interest

here are a few quick comparisons using the same base facts, changing only one variable at a time.

1. Simple vs. compound interest (same rate)

Same inputs as our main example, but change compounding:

• Principal: $25,000
• Start: 2023‑03‑02
• End: 2024‑09‑15
• Annual rate: 12%
• Method A: Simple interest
• Method B: Monthly compounding (our baseline example)

Approximate outcomes:

Difference due to compounding over ~18.5 months: ≈ $381.

If you’re checking someone else’s calculation, this is a useful early diagnostic:

• If their number is close to $4,600–$4,700, they may be using simple interest.
• If it’s closer to $5,000, they’re likely compounding (or using a slightly different day‑count).

2. Different annual rates (8%, 12%, 18%)

Keep everything else the same as the baseline (monthly compounding, same dates), and just change the annual rate:

Related reading

• Why deadlines results differ in Canada

• Worked example: deadlines in New York

• Emergency deadline checklist for Canada

• Why we started the DocketMath blog