The simple interest formula
Simple interest is I = P × R × T ÷ 100 — principal times rate times time. Nothing is reinvested, so the interest earned each year is identical and the total grows in a straight line.
It appears in short-term instruments: some car loans, many personal and bridging loans, and most fixed-term deposits under a year. Anything long-term almost always compounds instead.
Rearranging the formula
The relationship I = P × R × T ÷ 100 can be solved for any of its four terms. To find the rate, use R = 100 × I ÷ (P × T). To find the time, use T = 100 × I ÷ (P × R). To find the principal, use P = 100 × I ÷ (R × T).
This makes the formula useful in reverse: if you know what you paid in interest and over how long, you can recover the effective rate a lender actually charged you, which is not always the rate advertised.
Where simple interest still appears
Short-term instruments dominate: treasury bills, commercial paper, many bridging and payday loans, and most fixed deposits with terms under a year. Over a period shorter than one compounding cycle, simple and compound interest give the same answer anyway.
It also appears in car loans in some markets, and in the "flat rate" quotes used for consumer credit — where it makes a loan look considerably cheaper than an equivalent reducing-balance product at the same headline number.
The gap widens with time
Over one year the difference between simple and compound interest is negligible. Over five it is noticeable. Over thirty it is enormous: 10,000 at 8% simple interest yields 24,000 in interest over thirty years, while the same rate compounded annually yields roughly 90,600.
That divergence is the single most important idea in personal finance, and it cuts both ways — it works for you as a saver and against you as a long-term borrower.
Frequently asked questions
When would I prefer simple interest? +
As a borrower, always — it costs less than compound interest on the same rate and term. As a saver, the opposite is true.
How do I calculate simple interest per month? +
Divide the annual rate by twelve, or express the time in years as a fraction. Six months at 12% annual is the same as 0.5 years at 12%.
Which is better for a borrower? +
Simple interest, always, at the same nominal rate — you never pay interest on accumulated interest.