Simple Interest Calculator

Simple interest is charged on the original principal only, never on interest that has already accrued. That makes it grow in a straight line: the same amount is added every period, forever. It is the arithmetic behind most short-term consumer credit, car title loans, many bonds' coupon payments, and the classic textbook interest problem. It is also the honest baseline against which compound interest should be compared, because the gap between the two is the whole reason compounding matters. This calculator takes a principal, an annual rate and a period in years, and returns the interest earned or owed plus the final total. For anything longer than a few years, or anything where interest is reinvested rather than paid out, use compound interest instead — the difference over a decade is not small.

How this is calculated

Interest = principal × annual rate × time in years. Total = principal + interest. There is no exponent anywhere in the formula, which is exactly what makes it simple: each year adds the same amount as the year before.

Worked example on $10,000 at 5% for 3 years: interest is 10,000 × 0.05 × 3 = $1,500, and the total is $11,500. Every year contributes exactly $500, whether it is the first year or the thirtieth.

Partial years work the same way. Six months at 5% on $10,000 is 10,000 × 0.05 × 0.5 = $250. This is why simple interest is standard for short-term lending, where compounding would barely change the answer anyway.

Simple versus compound: how big is the gap?

On $10,000 at 5%, simple interest pays $500 a year indefinitely. Compound interest pays $500 in year one, $525 in year two, $551 in year three, and keeps accelerating.

Over 3 years the difference is small — $1,500 against $1,576. Over 30 years it is enormous: $15,000 against $33,219. The gap widens with both rate and time, which is why the distinction is trivial on a six-month loan and decisive on a retirement account.

Where you will actually meet simple interest

Short-term personal and payday loans, car title loans, and many auto loans in practice, since the balance is recalculated each month and interest does not compound on unpaid interest if you pay on time. Bond coupons are also simple: a bond paying 5% on $1,000 pays $50 a year regardless of how long you hold it.

It also appears in legal and tax contexts — statutory interest on late payments and judgment debts is frequently defined as simple. If a rate is quoted without stating a compounding frequency, simple interest is the safe assumption.

How to use the simple interest calculator

Enter the principal. The original amount lent, borrowed or invested. Simple interest never adds to this base.
Enter the annual rate. As a percentage. If the rate is quoted for a different period, convert it to annual first.
Enter the time in years. Decimals are fine — 0.5 for six months, 1.25 for fifteen months.

Frequently asked questions

What is the difference between simple and compound interest?

Simple interest is charged only on the original principal. Compound interest is charged on principal plus accumulated interest, so it grows faster over time — use our compound interest calculator to compare.

How do I calculate simple interest for months instead of years?

Convert the months to a fraction of a year and use that as the time input. Nine months is 0.75. The formula is unchanged: principal × rate × 0.75.

Is my savings account simple or compound interest?

Almost certainly compound — banks credit interest monthly or quarterly and it starts earning interest itself. Simple interest on savings is rare enough that if a product advertises it, that is usually a warning rather than a feature.

Which is better for a borrower?

Simple interest, always. It costs less than compound interest at the same nominal rate, and the gap grows with the length of the loan. When comparing loan offers, check the compounding frequency as well as the rate — two loans at 8% are not the same loan if one compounds monthly and the other does not compound at all.