Simple vs Compound Interest: What is the Difference and Which Is Better?
A mathematical and practical comparison of simple and compound interest, with formulas, examples, and guidance on when each applies.
Simple and compound interest are the two fundamental ways interest is calculated on loans, investments, and savings. The difference between them can amount to thousands or even millions of dollars over time, making it essential to understand which applies to your financial products. This comparison explains both calculation methods, shows real-world examples, and helps you identify which is being used in any financial situation.
Simple interest formula
Simple interest is calculated only on the original principal amount. The formula is: Interest = Principal x Rate x Time. For example, $10,000 at 5% simple interest for 3 years earns $10,000 x 0.05 x 3 = $1,500 in interest, for a total of $11,500. The interest is the same each year ($500) and does not compound.
Compound interest formula
Compound interest is calculated on the principal plus accumulated interest from previous periods. The formula is: Amount = Principal x (1 + Rate/n)^(n x Time), where n is the number of compounding periods per year. For $10,000 at 5% compounded annually for 3 years: $10,000 x (1.05)^3 = $11,576.25, earning $576.25 more than simple interest. Compounded monthly: $10,000 x (1 + 0.05/12)^36 = $11,614.72, earning $614.72 more.
Real-world comparison over 30 years
| Year | Simple Interest Total | Compound (Annual) | Compound (Monthly) |
|---|---|---|---|
| 1 | $10,500 | $10,500 | $10,511.62 |
| 5 | $12,500 | $12,762.82 | $12,833.59 |
| 10 | $15,000 | $16,288.95 | $16,470.09 |
| 20 | $20,000 | $26,532.98 | $27,126.40 |
| 30 | $25,000 | $43,219.42 | $44,677.04 |
Over 30 years, compound interest (monthly) earns $19,677 more than simple interest on the same $10,000 principal at the same 5% rate. This is why Einstein allegedly called compound interest the eighth wonder of the world.
Where each type applies
Simple interest is used in:
- Most auto loans (though they often use amortization which is similar)
- Some personal loans
- Some bonds (coupon payments)
- Short-term loans
Compound interest is used in:
- Savings accounts
- Investment accounts (stocks, mutual funds, retirement accounts)
- Credit cards (daily compounding)
- Mortgages (amortization uses compound interest principles)
- Student loans
Which is better?
For investments and savings, compound interest is dramatically better over time. For loans, simple interest is better for the borrower (you pay less total interest). Always check which interest calculation method applies to any financial product. Use the sevi.fun Compound Interest Calculator to model compound growth scenarios, and the sevi.fun Loan Calculator for amortizing loans that use compound interest principles.
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