Worked example: interest in Australia

When you’re dealing with overdue invoices, late-paid settlements, or judgment debts in Australia, a big practical question is: how much interest is this actually worth?

This worked example walks through a concrete scenario using the DocketMath interest calculator for Australia. You can follow the same pattern for your own matters, then plug the numbers into /tools/interest.

Example inputs

We’ll use a simple but realistic Australian scenario:

A business is owed $50,000 under a contract governed by New South Wales (NSW) law.
The invoice was due on 1 July 2023, but payment was only made on 15 January 2024.
The contract says that overdue amounts accrue interest at 10% per annum, simple interest, calculated on a 365‑day year.

We’ll also assume:

• No part-payments were made before 15 January 2024.
• Interest is not compounded.
• We’re not using statutory or court interest rates in this example, just the contractual rate.

These are the key inputs you’d enter into DocketMath:

1. Jurisdiction

   • Australia – NSW
   • Why it matters:

     • Some Australian states and territories have different statutory interest rules.
     • Courts may use different default rates or calculation conventions (e.g. 365 vs 366 days).
   • For this example, we’re using the contractual rate, but selecting the correct jurisdiction keeps the context right.

2. Principal amount

   • 50,000.00
   • This is the amount on which interest is calculated.
   • If your matter involves multiple invoices or stages, you’d usually break those into separate runs or use multiple line items if supported.

3. Interest type

   • Simple interest
   • Simple interest means:

     • Interest is calculated only on the original principal.
     • You don’t earn “interest on interest”.
   • In many Australian contractual and statutory contexts, interest is simple unless clearly stated otherwise.

4. Interest rate

   • 10% per annum
   • Expressed as a yearly rate.
   • DocketMath converts this into a daily rate using the selected day-count convention.

5. Day-count convention

   • Actual/365
   • Common for contractual interest in Australia:

     • Count the actual number of days.
     • Divide by 365 (not 366, even in a leap year).
   • If you’re dealing with court-ordered interest, check the specific rules for that court and period.

6. Start date

   • 1 July 2023
   • This is when interest starts accruing.
   • In practice, this is often:

     • The due date on an invoice, or
     • The date of default, or
     • A date specified in a judgment or order.

7. End date

   • 15 January 2024
   • This is when interest stops:

     • Usually the payment date, settlement date, or calculation date (if you’re projecting).

8. Compounding

   • No compounding (simple)
   • Even if compounding is allowed, many contracts and rules default to simple interest. Always check the wording.

9. Currency

   • AUD
   • For calculations, the currency symbol doesn’t change the math, but it keeps your reports clear.

A quick checklist before running the calculation:

• Correct Australian state/territory selected
• Principal matches the relevant invoice/amount
• Interest rate is clearly identified (contract vs statutory)
• Start and end dates align with the legal or contractual position
• Day-count convention matches the rule or practice you’re following

Example run

We’ll now walk through the calculation step by step, mirroring what DocketMath does behind the scenes.

Run the Interest calculator using the example inputs above. Review the breakdown for intermediate steps (segments, adjustments, or rate changes) so you can see how each input moves the output. Save the result for reference and compare it to your actual scenario.

1. Count the days

We need the number of days from 1 July 2023 (inclusive) to 15 January 2024 (exclusive, assuming interest stops on the payment date).

Breakdown:

• July 2023: 31 days
• August 2023: 31 days
• September 2023: 30 days
• October 2023: 31 days
• November 2023: 30 days
• December 2023: 31 days
• January 2024: 14 days (1st–14th, if interest stops before 15th)

Total days:

31 + 31 + 30 + 31 + 30 + 31 + 14 = 198 days

DocketMath does this automatically when you enter the dates.

2. Convert annual rate to daily rate

We’re using Actual/365:

• Annual rate = 10% = 0.10
• Daily rate = 0.10 ÷ 365

[ \text{Daily rate} = \frac{0.10}{365} \approx 0.0002739726 ]

That’s about 0.02739726% per day.

3. Apply simple interest formula

Simple interest formula:

[ \text{Interest} = \text{Principal} \times \text{Annual rate} \times \frac{\text{Days}}{365} ]

Substitute our numbers:

• Principal = 50,000
• Annual rate = 0.10
• Days = 198

[ \text{Interest} = 50{,}000 \times 0.10 \times \frac{198}{365} ]

First calculate the year fraction:

[ \frac{198}{365} \approx 0.54246575 ]

Then:

[ \text{Interest} = 50{,}000 \times 0.10 \times 0.54246575 ] [ \text{Interest} = 5{,}000 \times 0.54246575 ] [ \text{Interest} \approx 2{,}712.33 ]

So for this run, In DocketMath, this appears as:

• Principal: $50,000.00
• Interest period: 1 July 2023 – 15 January 2024
• Days: 198
• Rate: 10% p.a. (simple, Actual/365)
• Interest: $2,712.33
• Total (principal + interest): $52,712.33

4. How DocketMath presents it

In the DocketMath interface at /tools/interest, you’d typically see:

• A summary line with total interest and total payable.
• A breakdown showing:

  • Principal
  • Rate
  • Days
  • Day-count basis
  • Start/end dates
• Optionally, a step-by-step explanation (Explain++) for auditability, which can be exported or copied into a memo.

This structure makes it easier to:

• Justify your interest calculation to the other side.
• Check that the dates and rate align with your instructions.
• Update the end date and re-run if settlement is delayed.

Sensitivity check

The real value of a calculator is not just getting one answer; it’s seeing how the answer changes when inputs move. Here are a few quick sensitivity checks you can run in DocketMath.

To test sensitivity, change one high-impact input (like the rate, start date, or cap) and rerun the calculation. Compare the outputs side by side so you can see how small input shifts affect the result.

1. Changing the end date

Suppose payment is delayed to 1 March 2024 instead of 15 January 2024.

1. Keep everything the same.
2. Change End date to 1 March 2024.

Now recalculate the days:

• Original period: 1 July 2023 – 15 January 2024 = 198 days
• Additional days: 15 January 2024 – 1 March 2024

From 15 January (exclusive) to 1 March (exclusive):

• 15–31 January: 16 days
• February 2024 (leap year): 29 days

Extra = 16 + 29 = 45 days

New total days: 198 + 45 = 243 days

Interest:

[ \text{Interest} = 50{,}000 \times 0.10 \times \frac{243}{365} ]

[ \frac{243}{365} \approx 0.66575342 ]

[ \text{Interest} = 5{,}000 \times 0.66575342 \approx 3{,}328.77 ]

• Original interest: $2,712.33
• New interest: $3,328.77

Difference: about $616.44 for a 45‑day delay.

This lets you quickly answer questions like:

• “If we push settlement by six weeks, how much extra interest will we owe or receive?”
• “Is it worth negotiating the interest rate vs the settlement date?”

2. Changing the rate

Now keep the original dates but change the rate.

Scenario A – 8% p.a.

• Same 198 days, same principal.
• Annual rate = 8% = 0.08.

[ \text{Interest} = 50{,}000 \times 0.08 \times \frac{198}{365} ]

[ \text{Interest} = 4{,}000 \times 0.54246575 \approx 2{,}169.86 ]

Scenario B – 12% p.a.

• Annual rate = 12% = 0.12.

[ \text

Related reading

• Worked example: deadlines in New York

• Interest rule lens: Maine

• Common deadlines mistakes in Australia ]

• Why we started the DocketMath blog