Key takeaways
- Compound interest means you earn returns on your past returns, so growth speeds up over time.
- Time matters more than amount. At 7% a year, $300 a month from 25 to 35 (then nothing) grows to more by 65 than $300 a month from 35 to 65, despite a third of the contributions.
- The Rule of 72 gives a quick estimate: divide 72 by your return to see how many years it takes to double.
- These are illustrations with a steady assumed return. Real investment returns vary, and savings accounts earn much less.
Compound interest is the reason small, early savings can end up larger than big, late ones. Each year your money earns a return, and next year that return earns a return too. Early on the effect is barely noticeable. After a few decades it does most of the work.
How compounding works
Put $10,000 in an account earning 7% a year:
- After year one you have about $10,700: your $10,000 plus $700 of growth.
- In year two you earn 7% on $10,700, not on $10,000, so growth is a bit larger.
- Keep going and each year’s growth is bigger than the last. Left alone for 30 years at 7% compounded monthly, that $10,000 grows to about $81,165.
That last number is the key. Most of it is growth, not your deposit, and most of the growth arrives in the final decade.
Starting early vs. investing more
Compare two savers, each putting $300 a month into an account averaging 7% a year:
- Early Emma saves from 25 to 35, then stops contributing and leaves the money invested until 65.
- Later Liam starts at 35 and saves every month until 65.
| Early Emma | Later Liam | |
|---|---|---|
| Years contributing | 10 | 30 |
| Total contributed | $36,000 | $108,000 |
| Balance at 35 | $51,925 | $0 |
| Balance at 65 | $421,453 | $365,991 |
Emma contributes a third as much and still ends up with more. Her money had 30 extra years to compound, and that time was worth more than Liam’s extra 20 years of contributions.
And if Emma simply kept going from 25 all the way to 65? She’d have about $787,444, roughly double what either path gets on its own.
The Rule of 72
For a quick mental estimate, divide 72 by your annual return to get the approximate years it takes money to double:
| Annual return | Years to double (Rule of 72) |
|---|---|
| 3% | About 24 |
| 4% | About 18 |
| 6% | About 12 |
| 8% | About 9 |
| 9% | About 8 |
It works in reverse too. Inflation of 3% cuts the buying power of cash in half in about 24 years.
What makes compounding work harder
- Time. The single biggest factor. Starting five years sooner often matters more than saving a bit more each month.
- Rate of return. Small differences add up. Saving $300 a month for 40 years grows to about $787,444 at 7% but only $457,806 at 5%.
- Fees. A 1% annual fee comes out of your return every year, so it compounds against you. Low-cost index funds keep more of the growth.
- Staying invested. Withdrawing early, or selling during downturns and missing the recovery, interrupts compounding.
- Taxes. Tax-advantaged accounts like a 401(k) or IRA let growth compound without annual tax drag. See Roth vs. traditional IRA.
A note on assumptions
These examples use a steady 7% return to show the math. That’s a common planning assumption for long-term, stock-heavy portfolios, but real returns jump around: some years are strongly positive, some are negative. A high-yield savings account or CD gives you a steadier, much lower rate with no risk of loss. The principle is the same either way. The earlier money starts compounding, the more of your final balance comes from growth instead of your own contributions.
What to do with this
- If you’re early in your career, even a small automatic contribution now can outgrow a larger one started a decade later. Start with at least your full 401(k) employer match.
- If you’re starting later, you haven’t missed out. You’ll need to contribute more, but compounding still works for the decades ahead. Increasing contributions by 1% a year is an easy way to catch up.
- Automate it. Contributions that happen automatically don’t depend on remembering or feeling like it.
The compound interest calculator shows year-by-year growth with any contribution schedule and compounding frequency. To project a portfolio with regular investing, use the investment calculator. To work backward from a target, use the savings goal calculator.