Simple Interest vs Compound Interest
When does compounding beat simple interest? Here's the math, with break-even years and worked examples.
Simple interest pays a flat rate on the original principal. Compound interest pays on the principal plus all previously accumulated interest, which is why long horizons favor compounding so dramatically. The break-even is at year 1; from year 2 onward, compounding always wins, and the gap grows exponentially.
Key Differences
| Aspect | Simple Interest Calculator | Compound Interest Calculator |
|---|---|---|
| Formula | I = P × r × t | A = P(1 + r/n)^(nt) |
| On $10,000 at 7%, 30 yrs | $31,000 (interest $21k) | $76,123 (interest $66k) |
| Better for borrower | Yes — pays less | No — pays more |
| Better for investor | No | Yes — earns far more |
| Time horizon impact | Linear | Exponential |
When to use Simple Interest Calculator
- Short-term loans (auto loans, some personal loans)
- Bonds with simple interest payouts
- Quick estimates where the difference is < 5%
When to use Compound Interest Calculator
- Retirement accounts (401k, IRA, Roth)
- CDs and savings accounts
- Long-term debt where the lender benefits (credit cards)
- Any decade-plus investment horizon
Frequently Asked Questions
When does compound interest "beat" simple interest?
Always after year 1, but the gap is small at first. By year 10 at 7%, compound earns 30% more than simple. By year 30, it earns 145% more.
Is compound interest legal on consumer loans?
Yes. Most consumer loans (mortgages, credit cards, student loans) compound interest. Federal student loans capitalize unpaid interest at specific events (deferment ending, repayment beginning).
How can I make compound interest work for me?
Start early. A $5,000 contribution at age 25 grows to $77,000 by age 65 at 7% compound. The same $5,000 at age 45 grows to only $19,000.