Simple interest: definition, formula, examples, and calculator

Updated 12 Mar 2026Content was accurate at the time of publication

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.

What is simple interest?

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:

I = interest earned or payable
P = principal (initial amount)
r = annual interest rate (decimal)
t = time in years

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.

Simple interest formula (and key terms)

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:

Months to years: t = months ÷ 12
Days to years (ACT/365): t = days ÷ 365
Note: 30/360 day-count is used in some contracts; check the contract or product terms

Key derived formulae:

Interest: I = P × r × t
Amount: A = P + I

How to calculate simple interest — step by step

Follow these steps to compute simple interest correctly:

Identify the principal P (the starting amount).
Convert the annual rate from percent to decimal: r = rate% ÷ 100.
Convert the time period to years:
- For months: t = months ÷ 12
- For days: t = days ÷ 365 (or 30/360 if specified in the contract)
Substitute into I = P × r × t.
Compute total payable: A = P + I.
Show intermediate steps in your calculation for verification.

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.

Worked examples (with AUD values)

Example 1 — Multi-year loan (simple interest)

Principal: P = $1,000
Rate: 5% p.a. → r = 0.05
Time: 3 years → t = 3

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)

Principal: P = $12,000
Rate: 6% p.a. → r = 0.06
Time: 9 months → t = 9/12 = 0.75 years

Calculation: I = 12,000 × 0.06 × 0.75 = $140 Total payable: A = 12,000 + 540 = $12,540

Example 3 — Daily interest (ACT/365)

Principal: P = $10,000
Rate: 4.2% p.a. → r = 0.042
Time: 45 days → t = 45/365 ≈ 0.1233

Calculation: I = 50,000 × 0.042 × 0.1233 ≈ $159.59 Total payable: A ≈ $10,259.59

Example 4 — Short-term promissory note

Principal: P = $10,000
Rate: 3% p.a. → r = 0.03
Time: 60 days (ACT/365) → t = 60/365 ≈ 0.1644

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.

When simple interest is used

Simple interest commonly appears in:

Short-term promissory notes and bills of exchange (see promissory note)
Some commercial paper or business notes where interest is fixed and non-compounding
Specific term deposits or fixed-term arrangements that explicitly state simple interest — check product terms and conditions (see term deposits)
Some lease or loan contract variants where interest is calculated on the original amount — though many vehicle and equipment finance products use reducing balance or more complex structures (see novated lease and finance lease)

Many consumer loans and mortgages use a reducing-balance method or compound interest for unpaid balances. Always check the contract and repayment schedule.

Simple vs compound interest

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.

Common mistakes and pitfalls

Mixing time units: applying a monthly rate to a years-based formula without converting months to years.
Confusing flat-rate advertising with true simple interest: many "flat" quotes misrepresent the effective cost.
Ignoring day-count conventions: using 365 vs 360 can change short-term interest slightly but meaningfully for large principals.
Applying simple interest to amortising loans: the simple formula is not correct for loans with regular principal-and-interest repayments. For amortising loans use a repayment schedule (see loan repayments).
Forgetting to convert percent to decimal: 5% becomes 0.05.

Quick checklist before you trust a simple-interest calculation:

Have you converted months/days to years correctly?
Is the rate specified per annum, and did you convert % to decimal?
Does the contract use ACT/365 or 30/360?
Is the instrument amortising or non-amortising?

Tax and regulatory notes

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.

FAQ

What is the simple interest formula?

I = P × r × t, where P is principal, r is annual rate (decimal) and t is time in years.

How do you calculate simple interest for months or days?

Convert time to years: months ÷ 12 or days ÷ 365 (ACT/365) then apply I = P × r × t.

How is simple interest different from compound interest?

Simple interest charges only on principal; compound charges on principal plus accumulated interest, so compound grows faster.

Do banks use simple interest?

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.

What is a flat-rate loan — is that the same as simple interest?

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.

How is interest taxed — do I declare simple interest on my tax return?

Interest received is generally assessable income. For tax implications see the ATO page and consult a tax professional.

Can simple interest be used for loan repayments?

Only for non-amortising loans or agreements that specify interest-only payments; amortising repayments require a different calculation method.

How do I convert a monthly rate to an annual simple interest rate?

Multiply the monthly rate by 12 (for example, 0.5% monthly × 12 = 6% p.a.). For compound equivalents use compounding formulas.

Key takeaways

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.