Simple interest is interest calculated only on the original amount you borrowed or invested (the principal), not on accumulated interest. It's straightforward, predictable and often used for short-term instruments where interest doesn't compound.
Simple interest charges interest only on the principal amount, which makes it easier to calculate than compound interest. The formula is:
I = P × r × t
Where:
The total amount payable or accumulated is:
A = P + I
Simple interest differs from systems that accrue on previously earned interest. See the compound interest section below for a detailed comparison.
Each variable in the simple interest formula requires precise interpretation:
Principal (P): the initial amount, for example $1,000. Use AUD ($) in all calculations.
Rate (r): the annual nominal rate expressed as a decimal. Convert percentage to decimal: 5% becomes 0.05. If the rate is monthly, convert to an annual equivalent before applying the formula.
Time (t): measured in years. Convert months or days to years using these conversions:
Key derived formulae:
Follow these steps to compute simple interest correctly:
For partial-year convenience, note that 9 months equals 9/12 = 0.75 years, and 45 days equals 45/365 ≈ 0.1233 years.
Day-count conventions matter for short durations. Always check whether the contract or product terms specify ACT/365 or 30/360.
Example 1 — Multi-year loan (simple interest)
Calculation: I = 5,000 × 0.05 × 3 = $150 Total payable: A = 5,000 + 750 = $1,750
You would pay $150 interest on $1,000 at 5% p.a. for 3 years. Total repayable: $1,750.
Example 2 — Fractional year (months)
Calculation: I = 12,000 × 0.06 × 0.75 = $140 Total payable: A = 12,000 + 540 = $12,540
Example 3 — Daily interest (ACT/365)
Calculation: I = 50,000 × 0.042 × 0.1233 ≈ $159.59 Total payable: A ≈ $10,259.59
Example 4 — Short-term promissory note
Calculation: I ≈ 10,000 × 0.03 × 0.1644 ≈ $19.32 Total payable: $10,049.32
These examples show how small differences in time or rate change final interest in short-term situations.
Simple interest commonly appears in:
Many consumer loans and mortgages use a reducing-balance method or compound interest for unpaid balances. Always check the contract and repayment schedule.
The key difference is that simple interest charges only on the principal, while compound interest charges on principal plus previously accrued interest. Compound therefore grows faster.
Consider the same principal, rate and time:
| Example | Principal ($) | Rate p.a. | Time (yrs) | Simple interest I ($) | Compound amount A ($) |
|---|---|---|---|---|---|
| Case A | 10,000 | 5% | 3 | 1,500 → A = 11,500 | A ≈ 11,576.25 |
With compound interest, the total is approximately $16.25 higher than simple interest.
Whenever interest is credited and left to earn interest again (compounding periods ≥ 1), compound will exceed simple, with the gap widening as rate, time and compounding frequency increase.
Flat-rate loans vs simple interest vs reducing-balance:
Flat-rate loans sometimes advertise a flat percentage on original principal for the loan term, but repayments are often presented in a way that understates the effective reducing-balance cost. Reducing-balance loans compute interest on the outstanding balance after each repayment; effective cost is often higher than the flat headline.
To compare, convert flat and simple presentations into an annual percentage rate (APR) or use an amortisation schedule. For practical comparisons, set up side-by-side amortisation to see true cost — this is why calculators and clear disclosure matter.
Quick checklist before you trust a simple-interest calculation:
Interest income and tax: Interest you receive (for example, from deposits or notes) is generally assessable income. If you pay interest on borrowings for income-producing purposes, you may be able to claim deductions. For details, see the ATO page on interest income and consult a tax adviser.
Consumer protection and information: For consumer-facing guidance on how interest rates and loan costs work, see ASIC/Moneysmart. For context on how official interest rates influence markets and policy, see the RBA explainer on how interest rates work.
I = P × r × t, where P is principal, r is annual rate (decimal) and t is time in years.
Convert time to years: months ÷ 12 or days ÷ 365 (ACT/365) then apply I = P × r × t.
Simple interest charges only on principal; compound charges on principal plus accumulated interest, so compound grows faster.
Some short-term instruments or business notes use simple interest; most retail loans use reducing-balance or compound methods. Check the loan schedule or product terms.
Not always. Flat-rate often applies a percentage to the original principal for the term but repayments may be calculated differently. Compare effective rates versus reducing-balance.
Interest received is generally assessable income. For tax implications see the ATO page and consult a tax professional.
Only for non-amortising loans or agreements that specify interest-only payments; amortising repayments require a different calculation method.
Multiply the monthly rate by 12 (for example, 0.5% monthly × 12 = 6% p.a.). For compound equivalents use compounding formulas.
Simple interest is a straightforward method for calculating interest on the original principal and is most useful for short-term, non-amortising instruments. Always confirm time units, day-count convention (ACT/365 vs 30/360), and whether the product uses simple, flat or reducing-balance calculations. For example, you can use a personal loan calculator to estimate costs, and then verify by hand using the worked steps shown here.
This article is general information only and is not legal, tax or financial advice.