About the Simple Interest Calculator
Simple interest is charged only on the original principal, never on interest already earned, which makes it the most predictable form of interest there is. This simple interest calculator applies the standard SI = (P × R × T) ÷ 100 formula and shows the interest, the total amount payable and the interest accruing per year, with the substitution written out so you can check it against a lender's statement or reproduce it in an exam. Time accepts fractions, so a fifteen-month arrangement is entered as 1.25 rather than being rounded to a whole year.
How to use the Simple Interest Calculator
- 1
Enter the Principal — the original sum lent, borrowed or deposited.
- 2
Enter the Annual rate as a percentage, matching the period used for time.
- 3
Enter the Time in years, using decimals for part years such as 0.5 for six months.
- 4
Read the interest, total amount and yearly interest figures, then check the Working panel for the substituted formula.
What people use it for
Short-term personal and family loans
Informal loans between people are almost always simple interest. Lending 10,000 at 7.5% for three years produces 2,250 in interest and a 12,250 repayment, which is easy to write into an agreement.
Checking a fixed deposit that pays out
A deposit that credits interest to a separate account rather than reinvesting it earns simple interest. Use this to verify the quarterly or annual payout against the certificate.
School and exam practice
The working panel shows the full substitution, so students can compare their own layout against the standard method rather than only checking the final answer.
Where simple interest genuinely applies
The formula is SI = (P × R × T) ÷ 100, with the division by 100 converting the percentage into a decimal. On 10,000 at 7.5% for 3 years the interest is (10,000 × 7.5 × 3) ÷ 100 = 2,250, and the total repayable is 12,250. Because the base never changes, the interest accrues in a straight line: exactly 750 per year in that example, which is the figure the calculator reports as interest per year. The same 10,000 under annual compounding at 7.5% would produce about 2,423 over three years, so the gap is modest over short horizons and widens rapidly beyond ten years. Simple interest is common on short-term personal lending, on certain car and consumer finance agreements, on Treasury-style discount instruments and on fixed deposits that pay interest out rather than reinvesting it. Court-awarded interest on damages is also frequently calculated on a simple basis. What it is almost never used for is credit cards or mortgages, which compound.
The flat rate trap
Some lenders quote what they call a flat rate, which is simple interest applied to the full original principal for the whole term even though you are repaying in instalments. This is misleading, because after the first payment you no longer have the full principal, yet you are still charged as if you did. A 100,000 loan over 5 years at a 6% flat rate produces 30,000 of interest and total repayments of 130,000, which sounds reasonable until you convert it. The equivalent reducing-balance rate is close to 10.9% — nearly double the quoted figure. The rough approximation is that a flat rate corresponds to a reducing-balance rate of roughly twice as much for a typical instalment loan, converging on exactly double as the term lengthens. Whenever a quote seems unusually cheap, ask explicitly whether the rate is flat or reducing-balance, and compare offers on total amount repayable. These figures are estimates for comparison and not financial advice.
Tips
- Keep rate and time on the same footing: a monthly rate needs time in months, or convert the rate to an annual figure first.
- For any term beyond about ten years, run the compound interest calculator alongside this one — the divergence becomes substantial.
- Interest per year is the quickest way to sanity-check a lender's schedule; multiply it by the term and it should match the total interest.