Calculate simple interest earned or owed on a principal amount over a given time period.
This calculator computes simple interest — interest calculated only on the original principal amount, not on any previously accumulated interest. It shows the total interest earned (or owed) and the final amount at the end of the period. Simple interest is used in some short-term loans, bonds, and promissory notes, and is conceptually straightforward to understand and verify.
Simple interest is calculated on the principal only — the interest earned in each period does not add to the balance for the purpose of calculating future interest (unlike compound interest). This makes the calculation straightforward.
Formula: Interest = Principal × Annual Rate × Time. Final Amount = Principal + Interest.
Example: An A$10,000 principal at a 5% annual interest rate over 3 years: Interest = A$10,000 × 0.05 × 3 = A$1,500. Final Amount = A$11,500. In contrast, if interest were compounded annually at the same rate over the same period, the final amount would be A$10,000 × (1.05)3 = A$11,576.25 — A$76.25 more than with simple interest, because compound interest earns interest on accumulated interest.
Simple interest is less commonly used than compound interest for consumer financial products in Australia, but it appears in specific contexts: short-term personal loans, some hire-purchase agreements, and bond interest calculations use simple interest concepts. The distinction between simple and compound interest matters most over longer time horizons or at higher interest rates — over short periods (e.g., a few months), the difference is small. Most Australian savings accounts, home loans, and investment products use compound interest, where interest is credited periodically (daily, monthly, or annually) and added to the balance, so future interest is calculated on a growing balance. Understanding the difference between simple and compound interest is also useful for interpreting advertised rates: a flat fee stated as a percentage of the original loan amount (common in some short-term "payday" loan products) is mathematically equivalent to a simple interest rate — but when expressed as an effective annual percentage rate on the declining balance (as required for comparison under the National Consumer Credit Protection Act), the equivalent rate is much higher, which is why these products are regulated. The Australian Securities and Investments Commission (ASIC) requires lenders to disclose comparison rates, which incorporate the effect of fees and compounding frequency into a single annualised rate for consumer comparison purposes.
Simple interest is calculated only on the original principal, so the interest amount is the same every period. Compound interest is calculated on the principal plus all previously accumulated interest, so the balance (and the interest earned) grows faster over time. For savings and investments, compound interest is better for the saver; for loans, compound interest costs more for the borrower. Most financial products in Australia use compound interest.
Simple interest appears in some specific contexts: short-term personal loans and some hire-purchase products may state interest as a flat percentage of the original loan amount (which is mathematically simple interest). Some government bonds and fixed-income securities calculate periodic interest payments based on the face value (a form of simple interest on each coupon period). For everyday savings accounts, home loans, and personal loans, compound interest is almost universally used.
A "flat rate" loan charges interest as a fixed percentage of the original principal for the full term, regardless of how much you have repaid. Because you are paying interest on the full original amount even as your balance reduces (through repayments), the effective interest rate on the declining balance is significantly higher than the stated flat rate — roughly double for a loan repaid in equal instalments over its term. This is why the National Consumer Credit Protection Act requires Australian lenders to disclose a comparison rate that reflects the true effective rate.
Yes — if a short-term loan charges a fixed fee or interest calculated on the original amount (not on the declining balance), you can use this calculator to determine the total interest cost. Enter the loan amount as the principal, the annual equivalent rate as the interest rate, and the loan period in years (e.g., 0.5 for 6 months). This helps you understand the total cost and compare it with the interest that would apply to a standard compound-interest loan.
With simple interest, the same fixed dollar amount of interest is added every period (Principal × Rate), so the balance grows in a straight line over time. With compound interest, the interest each period is a percentage of a growing balance — so the dollar amount of interest increases each period, and the balance grows at an accelerating rate (exponentially). Over short periods at low rates, the difference is small; over decades at higher rates, the difference is very large.
Partially. Some term deposits pay simple interest — a fixed percentage of the principal — at maturity or periodically, without reinvesting the interest. If that is how your term deposit is structured, this calculator gives the correct interest figure. However, if interest is compounded within the term deposit (reinvested), use our Compound Interest Calculator instead. Check the product disclosure statement for your term deposit to confirm how interest is calculated and paid.
Disclaimer: The information and figures provided on this page are for educational and illustrative purposes only and do not constitute financial advice. Simple interest calculations are a simplified model — most real-world financial products in Australia use compound interest and may include fees not captured here. Always check the product disclosure statement, comparison rate, and terms and conditions of any financial product before committing. Consult a qualified financial adviser for advice specific to your circumstances.