When is simple interest used?

Short-term loans, many car finance agreements, some bonds and certain fixed deposits. Most savings accounts, credit cards and mortgages compound instead.

How do I handle months rather than years?

Divide months by 12. Nine months is 0.75 years.

How much difference does compounding make?

At 5% over 3 years the difference is small — about $76 on $10,000. Over 30 years it becomes enormous, which is why the compound interest calculator is worth using for anything long term.

Simple Interest Calculator

Simple interest is charged only on the original amount, never on interest already accrued. It is used for short-term personal loans, some car finance and many fixed deposits.

Fill in the fields above and select Calculate to see your result and the full working.

Please note: Financial calculators produce estimates using the figures you enter. They exclude fees, taxes, insurance and rate changes unless a field asks for them. They are not financial advice — confirm any significant decision with a qualified adviser or your lender.

Formula used

I = P × r × t, where r is the annual rate as a decimal and t is time in years

How to use this calculator

Enter the principal — the amount borrowed, deposited or invested.
Add the annual interest rate as a percentage.
Enter the time in years; use decimals for part-years (six months = 0.5).
Calculate to see the interest and the final balance.

Example calculation

$10,000 at 5% for 3 years:

I = 10,000 × 5 × 3 ÷ 100

I = $1,500 in interest

Final balance = $11,500.

What does this result mean?

Because interest never earns interest, simple interest always produces a smaller total than compound interest over the same period and rate. If a lender quotes a flat rate, the effective annual rate you actually pay is usually higher, because you repay part of the principal each month while still paying interest on the full original amount.

Frequently asked questions

When is simple interest used?: Short-term loans, many car finance agreements, some bonds and certain fixed deposits. Most savings accounts, credit cards and mortgages compound instead.
How do I handle months rather than years?: Divide months by 12. Nine months is 0.75 years.
How much difference does compounding make?: At 5% over 3 years the difference is small — about $76 on $10,000. Over 30 years it becomes enormous, which is why the compound interest calculator is worth using for anything long term.

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