Breakdown
How it works
Simple interest never earns interest on interest, so it grows in a straight line. Compound interest grows exponentially. For short periods the difference is small; over decades it is enormous.
Formula
Interest = P × R × T (R as a decimal, T in years)
Total = P + Interest
Compound comparison: P × (1 + R)ᵀ
Worked example
10,000 at 5% for 3 years: simple interest 1,500 (total 11,500); compounded annually 1,576 (total 11,576).
Frequently asked questions
What uses simple interest in practice?
Short-term personal loans in some markets, "flat rate" car finance in the GCC and Asia, treasury bills, late-payment interest and most school maths questions.
How do I convert a flat rate to a real APR?
Roughly double it: a 3% flat rate over 4 years ≈ 5.6% reducing-balance APR, because you pay interest on the full principal even as you repay it.
Is simple interest better for a borrower?
For the same quoted rate, yes — but lenders quoting flat rates set them lower to look attractive. Always compare the APR.