Simple Interest Examples Australia 2026: Formula, Loans and Savings Scenarios

The simple interest formula

Simple interest is calculated only on the original principal. The formula is:

Interest = Principal x Rate x Time

If you invest or borrow $10,000 at 6% simple interest for 2 years, the interest is:

$10,000 x 0.06 x 2 = $1,200

The final amount is $11,200.

Use the Simple Interest Calculator to test your own amount, rate and term.

Why simple interest is useful

Simple interest is not the right model for every financial product, but it is useful when interest does not compound or when you need a quick estimate.

Common uses include:

• Short-term private loans.
• Basic interest-only calculations.
• Quick savings estimates before compounding.
• Comparing a fixed fee against an interest charge.
• Explaining the difference between simple and compound interest.

Example 1: short-term loan

Assume a $20,000 loan at 8% simple interest for 18 months.

Time must be expressed in years, so 18 months is 1.5 years.

Interest = $20,000 x 0.08 x 1.5 = $2,400

The total repayment before fees would be $22,400.

This is useful for a quick estimate, but most regulated consumer loans have fees, compounding or amortising repayments. For a home loan, use the Mortgage Repayment Calculator instead.

Example 2: savings goal

Assume $15,000 in savings at 4% simple interest for 3 years.

Interest = $15,000 x 0.04 x 3 = $1,800

The final balance is $16,800 before tax.

For a deposit goal, simple interest gives a rough estimate, but real savings accounts may compound monthly and your contributions may change. Use the Deposit Savings Calculator for a more practical deposit timeline.

Example 3: interest-only property debt

Investors often use simple interest as a quick approximation for annual interest cost:

Annual interest = loan amount x interest rate

A $600,000 interest-only loan at 6.25% has annual interest of:

$600,000 x 0.0625 = $37,500

That annual interest estimate can then be included in a rental-yield or negative-gearing model.

Use the Rental Yield Calculator to compare rent, vacancy, land tax, strata and interest.

Simple interest vs compound interest

The key difference is what happens after the first interest period.

With simple interest, interest is always calculated on the original principal.

With compound interest, interest is added to the balance and future interest is calculated on the larger amount.

For long timeframes, compound interest grows faster. That is why simple interest can understate investment growth and over-simplify loan costs.

Use the Compound Interest Calculator when interest is reinvested or capitalised.

Simple interest vs amortised repayments

Home loans are usually amortised, not simple-interest loans. A mortgage repayment includes both interest and principal. Early repayments contain more interest; later repayments contain more principal.

That means the simple annual interest estimate is useful for a quick check, but not enough to calculate a full 30-year repayment schedule.

For mortgages, model:

• Loan amount.
• Interest rate.
• Loan term.
• Repayment frequency.
• Extra repayments or offset balance.

Then use the amortisation table to see how the loan balance falls over time.

Common mistakes

Mistake 1: forgetting to convert months into years

If the term is 6 months, time is 0.5 years. If the term is 18 months, time is 1.5 years.

Mistake 2: using percentage instead of decimal

Six percent is 0.06 in the formula, not 6.

Mistake 3: using simple interest for compounding products

Savings accounts, investments and some debts can compound. Simple interest may be too low for long-term investment growth and too simplistic for many loans.

Mistake 4: ignoring fees and tax

Simple interest does not include account fees, loan fees, tax on interest income or inflation.

Best next step

Use simple interest for a fast estimate, then choose the calculator that matches the product:

• Simple Interest Calculator
• Compound Interest Calculator
• Mortgage Repayment Calculator
• Deposit Savings Calculator

General information only. Check the product disclosure statement or loan contract for how interest is actually calculated.