Meet Alex and Jordan. They both save $300 per month at a 5% annual return. The only difference? Alex chose a simple interest account, Jordan chose compound. After 20 years, how much does that one choice matter?
Alex vs Jordan: The 20-Year Result
Alex (Simple Interest)
$108,000
$72K principal + $36K interest
Jordan (Compound Interest)
$123,310
$72K principal + $51K interest
Same amount, same rate — $15,310 difference. With compound interest, your earnings generate their own earnings.
Simple vs Compound: One-Line Summary
- Simple interest: calculated on the original principal only. Interest stays flat.
- Compound interest: calculated on principal + accumulated interest. Grows faster over time.
How the Gap Widens Over Time
$300/month at 5% annual return:
| Period | Total Deposited | Simple | Compound | Difference |
|---|---|---|---|---|
| 5 years | $18,000 | $20,250 | $20,400 | $150 |
| 10 years | $36,000 | $45,000 | $46,594 | $1,594 |
| 20 years | $72,000 | $108,000 | $123,310 | $15,310 |
| 30 years | $108,000 | $189,000 | $249,677 | $60,677 |
The first 5 years show barely any difference. But past 10 years, the compound effect accelerates dramatically. By 30 years, the gap exceeds $60,000.
The Rule of 72: When Does Your Money Double?
A quick formula to estimate doubling time with compound interest:
72 ÷ annual return (%) = years to double
Example: 5% return → 72 ÷ 5 = ~14.4 years
| Annual Return | Time to Double |
|---|---|
| 3% | ~24 years |
| 5% | ~14.4 years |
| 7% | ~10.3 years |
| 10% | ~7.2 years |
How to Maximize Compound Interest
① Start early. Starting at 25 instead of 35 — with the same monthly amount — leads to dramatically more wealth. Time is compound interest's greatest weapon.
② Don't withdraw returns. Reinvesting interest is what makes compounding work. Withdrawing turns it back into simple interest.
③ Check compounding frequency. Monthly compounding beats annual. The shorter the cycle, the faster your money grows.
④ Be consistent. Small regular contributions beat irregular lump sums. The compound machine runs on consistency.
Calculate Your Savings Growth
Enter your monthly contribution, interest rate, and time period to compare simple vs compound interest results. Try different scenarios to build your ideal savings plan.