The Power of Compound Interest
Compound interest is the single most important force in reaching financial independence: it's what happens when your investment returns start earning returns of their own. Every FIRE plan — every projection in our calculators — runs on it. Understanding why it accelerates over time is what turns saving from a chore into a strategy.
Key takeaways
- Compounding means your gains earn gains — growth speeds up over time.
- The Rule of 72 estimates doubling time: 72 ÷ return rate.
- The earliest dollars you invest do the most work.
- Time in the market usually beats trying to time the market.
Simple vs compound growth
With simple interest, you'd earn the same dollar amount every year. With compound interest, each year's growth is calculated on a bigger balance — last year's gains included. That small difference becomes enormous over decades.
Invest $10,000 at 7% a year. Year one adds $700. But by year 20 your balance is around $38,700, and that year alone adds roughly $2,700 — nearly four times the first year's growth, from the same untouched $10,000. You didn't add a cent; the returns compounded.
The Rule of 72
You don't need a spreadsheet to estimate compounding. The Rule of 72 tells you roughly how long money takes to double:
At 7% a year, money doubles about every 10 years (72 ÷ 7 ≈ 10.3). So $50,000 today becomes ~$100,000 in a decade, ~$200,000 in two, and ~$400,000 in three — with no new contributions. That doubling-on-doubling is the whole game.
Why starting early wins
Because each doubling builds on the last, your earliest dollars matter most — they get the most doublings. Consider two savers, both investing $5,000 a year at 7%:
- Early Ellie invests from 25 to 35 (10 years, $50,000 total), then stops and never adds again.
- Late Liam invests from 35 to 65 (30 years, $150,000 total).
At 65, Ellie often ends up with more money than Liam — despite investing a third as much — because her early contributions had 40 years to compound. This is why the FIRE community obsesses over investing aggressively in your 20s and early 30s.
What compounding needs from you
- Time. The non-negotiable ingredient. Start now, even small.
- Consistency. Regular contributions add fuel; automate them so you never skip.
- Reinvested returns. Dividends and gains must stay invested to compound — don't skim them off.
- Low fees. A 1% annual fee compounds against you; over decades it can cost a large share of your final balance. Low-cost index funds keep more of the growth yours.
- Patience through downturns. Selling in a crash breaks the chain. Staying invested lets the recovery compound.
The flip side: compounding costs
The same math works against you with high-interest debt. Credit-card balances compound in the lender's favor at 20%+ — doubling what you owe in under four years by the Rule of 72. That's why paying off high-interest debt is often the highest "return" available before you invest.
Frequently asked questions
What is compound interest?
Compound interest is when your investment returns themselves earn returns. Instead of growing by the same amount each year, your balance grows by a percentage of an ever-larger total, so growth accelerates over time.
What is the Rule of 72?
The Rule of 72 estimates how long an investment takes to double: divide 72 by the annual return rate. At 7% a year, money doubles roughly every 72 divided by 7, or about 10.3 years.
Why does starting early matter so much?
Because compounding accelerates over time, the earliest dollars you invest have the most years to double and re-double. A dollar invested at 25 can become many times more by 65 than the same dollar invested at 40.
Related: What Is Coast FIRE? · How to Invest for FIRE · FIRE Calculator