Simple interest grows linearly — it is always calculated on the original principal, never on previously earned interest. The formula is: I = P × r × t, where P is the principal, r is the annual rate as a decimal, and t is the time in years. A $10,000 loan at 5% simple interest over 3 years accrues $1,500 — exactly $500 per year, every year.
Compare this to compound interest, where interest earns interest. The same $10,000 at 5% compounded annually for 3 years produces $1,576 — $76 more. The gap widens dramatically over longer periods. Over 30 years at 6%, compound interest generates more than double the final balance of simple interest.
Simple interest applies to most auto loans, many personal loans, short-term bonds, and U.S. Treasury bills. For savings products like CDs and high-yield savings accounts, interest always compounds — making the compound interest calculator the relevant tool for savers.
Is simple interest ever better than compound interest?
For borrowers, yes — you pay less total interest on a simple interest loan than a compound interest loan at the same rate and term. For savers and investors, compound interest is always preferable because your earnings generate additional earnings over time.
Which real-world loans use simple interest?
Most auto loans and many personal loans use simple interest. Your monthly payment is fixed, but the split between principal and interest shifts each month. Making extra payments reduces your principal faster — and since interest is recalculated daily on the remaining balance, you save meaningfully on total interest paid.
How do I calculate simple interest manually?
Multiply principal × annual rate × time in years. For $5,000 at 4% over 18 months: $5,000 × 0.04 × 1.5 = $300 in interest. Total repayment: $5,300. For periods shorter than a year, convert months to a decimal fraction (6 months = 0.5 years).
What’s the difference between simple interest and APR?
APR (Annual Percentage Rate) is a standardized rate that may include fees on top of the base interest rate. Simple interest is purely the base rate applied to principal over time, with no compounding. For mortgages and credit cards, APR includes additional costs, making it higher than the stated interest rate alone.