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How simple interest works

Simple interest is calculated only on the original principal. It never earns interest on previously earned interest, which is the single feature separating it from compound interest. Enter your principal, annual rate, and time in years to see the interest earned and the final balance.

The formula is the most direct in finance:

I = P × R × T

Where I is interest, P is principal, R is the annual rate as a decimal, and T is time in years. The final balance is simply P + I.

Worked example: $10,000 at 5% for 3 years produces 10,000 × 0.05 × 3 = $1,500 in interest, for a final balance of $11,500. Each year earns exactly $500 — no more, no less, regardless of how long the money sits.

Simple versus compound: the difference over time

Over short periods the two are close. Over long ones they diverge dramatically, because compound interest earns interest on interest while simple interest does not.

Take $10,000 at 5%. Under simple interest it earns $500 every single year forever. Under annual compounding it earns $500 in year one, $525 in year two, $551 in year three, and so on. After 10 years simple interest yields $5,000 while compounding yields about $6,289. After 30 years the gap is stark: $15,000 versus roughly $33,219 — more than double.

The practical lesson is directional. You want compound interest when you are the lender or saver, and simple interest when you are the borrower.

Where you actually encounter simple interest

It is less common than compound interest but appears in specific places:

• Most car loans — interest accrues on the outstanding balance without compounding, which is why paying early genuinely reduces total interest.
• Many personal loans and short-term consumer credit.
• Some bonds and certificates that pay interest out rather than reinvesting it.
• Court judgments and late-payment terms, where statutes often specify simple interest.
• Short-term business lending and bridging finance.

Savings accounts, credit cards, mortgages, and investment returns essentially all use compounding.

Rearranging the formula

Because I = PRT has four variables, knowing any three gives you the fourth:

• Principal: P = I ÷ (R × T)
• Rate: R = I ÷ (P × T)
• Time: T = I ÷ (P × R)

These are useful for checking a lender's figures. If you are told a $5,000 loan will cost $900 in interest over 2 years, then R = 900 ÷ (5,000 × 2) = 0.09, or 9% — which you can then compare directly against other offers.

Common mistakes

Mixing up time units. The formula expects T in years when R is annual. Six months is 0.5, not 6. Ninety days is 90/365, roughly 0.247.

Using the percentage instead of the decimal. R must be 0.05, not 5. Entering the percentage inflates the answer by a factor of one hundred.

Assuming a loan is simple interest because the lender said so. Verify how interest accrues and whether unpaid interest is capitalised. A loan described as simple interest can still compound if missed interest is added to principal.

Frequently Asked Questions