This is general information, not investment advice. The 7% return used below is illustrative, not a promise.
Albert Einstein supposedly called it the eighth wonder of the world. Whether or not he did, compound interest is the single idea that does the most quiet work in any savings plan.
Simple vs. compound
Simple interest is earned only on your original sum (the principal). Put $1,000 at 7% simple interest and you earn $70 every year — $2,100 over 30 years, leaving $3,100.
Compound interest is earned on the principal and on all the interest already accumulated — interest on interest. That same $1,000 at 7%, compounded annually, grows to about $1,967 in 10 years, $3,870 in 20 years, and $7,612 in 30 years — more than double the simple-interest result, per the SEC's compound-interest calculator. The gap widens because each year's gain is bigger than the last.
| Year | Balance ($1,000 at 7%) |
|---|---|
| 0 | $1,000 |
| 10 | $1,967 |
| 20 | $3,870 |
| 30 | $7,612 |
When a bank quotes an APY (annual percentage yield), it's showing the effective rate after compounding — and the more often interest compounds (daily beats monthly beats yearly), the higher that effective return.
The Rule of 72
A handy shortcut: divide 72 by your annual return to estimate the years to double your money. At 6% that's 12 years; at 9%, eight years; at 7%, about ten. FINRA notes the rule cuts the other way too — debt at 18% doubles in about four years (72 ÷ 18). It's an approximation, best between roughly 6% and 10%, but good enough for quick planning.
Why time beats money
The rate matters; the time horizon matters more. Consider two savers, both earning 7%:
- Early starter invests $2,400 a year (about $200/month) from age 25 to 35 — ten years, $24,000 total — then never adds another dollar. By 65, that grows to roughly $252,000.
- Late starter invests the same $2,400 a year from 35 to 65 — thirty years, $72,000 total, three times as much. By 65 they have about $227,000.
The early starter put in $48,000 less and still finishes ahead. The extra decade of compounding in their 20s did more than three times the contributions could later undo. That's why "start now" is the most valuable advice in personal finance.
When it works against you
The same math powers debt. Credit cards typically compound and charge 20%-plus APR. A $5,000 balance at 20%, left unpaid, roughly doubles to about $11,000 in four years (Rule of 72: 72 ÷ 20 ≈ 3.6 years). APR is the yearly cost of carrying a balance; paying only the minimum barely dents the interest, which is why balances feel like quicksand.
Practical takeaways
- Start as early as you can — even small amounts. Time is the lever you can't get back.
- Reinvest dividends and interest so the base keeps growing.
- Kill high-interest debt first — clearing a 20% balance is like earning 20%, risk-free.
- For savings, favor accounts that compound more frequently; for debt, the opposite.
Compound interest won't make anyone rich overnight — its power is precisely that it's slow and relentless. The earlier you let it start, the more of the work it does for you. For decisions specific to your situation, consult a qualified professional.
